The cost of accommodations can take a big chunk out of a vacation budget, particularly in major tourist cities. It's not unusual these days for large downtown hotels to charge from $100 to $200 a night, and that's not for the penthouse suite.

But clean, comfortable low-cost alternatives to high-priced rooms are available throughout this country -- and abroad. They are aimed at people, young and old, who enjoy traveling but don't want to spend a lot of money.

The alternatives include summer lodging in college and university dormitory rooms; an international network of YMCA hotels; the international youth hostel organization; and two home-stay programs, where travelers take a room in a private home for a nominal charge or none at all.

For example:

* A couple visiting the West Coast can rent a double room on the gorgeous hillside campus of the University of California at Santa Cruz for about $24 a night. True, as in many dorms, the bathrooms are down the hall. But some of the rooms have a good view of the Pacific Ocean, and a beautiful resort beach is only a few minutes away.

* Families who join INNter Lodging Co-op, one of the home-stay organizations, could plot a cross-country driving tour, paying only $5 a night per adult and 25 cents for each child for a bedroom and bath in private homes along their entire route. They may have to search for their hosts in unfamiliar neighborhoods, but the welcome should be friendly.

* At the American Youth Hostel facility in Arizona's Grand Canyon National Park, just a five-minute walk from the canyon's popular South Rim, the summer rate for a dorm bed is $7.25 per person. For that price, though, you will share a four-person bunk room and be expected to help briefly with morning cleanup chores.

Obviously, these kinds of accommodations are not for everyone. The traveler gives up a great many services provided in hotels and motels -- and some privacy. But for travelers with a certain spirit of adventure they offer both substantial savings and rewards no hotel can duplicate.

One young traveler from Washington, a 24-year-old woman, stayed two or three nights each in several private homes in Australia under an international program called Servas. She was treated like a member of the family, she says, getting an inside look at Australian life. Once, on a farm, she donned overalls and boots to help with the early-morning milking.

Among the alternatives:

* On-Campus Lodging: In recent years, colleges and universities have opened their student dormitories in the summer to travelers. Many colleges include the use of other campus facilities (such as the cafeteria and gymnasium) in what is usually a very low price.

Such dorm rooms (for singles, couples and families) are available on many prestigious and well-located campuses such as California's Santa Cruz. They generally are open to travelers of all ages.

The campuses usually have active summer programs, and visitors may share the dorms with graduate students, teachers returning for advanced study, delegates to educational and scientific conferences and tourists like themselves. A bustling cultural life of lectures, seminars, movies and concerts is often open to dorm guests.

Two teachers have compiled separate guides to dorm accommodations. Both say they make use of dorms themselves because it is a cheap way to travel on a teacher's salary.

They describe the typical college dorm room as a clean, safe, comfortable place to sleep. But there are no bellhops to carry luggage, guests are often expected to make their own beds, room service is nonexistent and the bath is probably down the hall.

The most extensive of the two guides is the "U.S. and Worldwide Travel Accommodations Guide," by John Jensen, a former professor at Orange Coast College in Costa Mesa, Calif. He has included about 275 U.S. campuses and another 230 in 25 foreign countries. His booklet sells for $7.95. To obtain a copy: Campus Travel Service, 1303 E. Balboa Blvd., Newport Beach, Calif. 9266l, (714) 675-9891.

The other guide, "On-Campus Lodging for Tourists," was compiled by Jess Miller, a high school counselor in Pellston, Mich. His booklet lists available facilities on only 150 U.S. campuses, but he provides more detailed descriptions of dorm amenities (such as linen service, TVs) and nearby tourist attractions. Miller's booklet sells for $8. To obtain a copy: Jess Miller Publishing Co., P.O. Box 370, Indian River, Mich. 49749, (616) 238-7116.

* The Y's Way: Back in 1978, the New York YMCA was looking for ways to fill its overnight rooms. Officials hit upon the idea of a cooperative network uniting "Ys" throughout the United States, Europe and Asia through a central reservation office in New York.

The results have been "tremendous," says Gerhard W. van den Bergh, executive director of "The Y's Way." This is the name given to the accommodations program, which is open to male and female travelers (couples included) of all ages. A pamphlet lists 68 YMCA "overnight centers" in the United States, 30 in Western Europe and nine in the Far East.

The average rate for a Y room, located mostly in major cities, is $18 to $20, and that includes the use of the sauna, swimming pool and other health and athletic facilities featured in most Ys. But don't count on a private bathroom.

A double room in New York's Vanderbilt Y, at 224 E. 47th St., is $28 a night; in Boston, the double rate of $35 at the Central Y includes breakfast; the single rate in Honolulu (where men and women must stay in separate rooms) is $17. The charge for a single at the excellent Tokyo Y (males only) is about $20, and guests are provided a kimono for their stay.

To obtain a listing of Y accommodations and reservation requirements: The Y's Way, 356 W. 34th St., New York City, N.Y. 10001, (212) 760-5856. Enclose a self-addressed stamped (40 cents) legal-size envelope.

* Servas: The first thing a Servas official tells an inquiring traveler is that Servas is not a cheap lodging association. It is a longstanding "peace" organization, says New York administrative assistant Maggie Wise, "furthering peace through international understanding."

This objective is pursued by facilitating short home-stays for travelers in 90 countries, principally the United States, Canada, Western Europe, Australia and New Zealand.

About 7,000 host families abroad and 1,800 in the United States welcome guests in their homes, generally for two nights, at no charge -- although longer stays can be arranged if host and guest desire. Guests are expected to join in family activities, sharing the chores. It is a way of learning about another culture, says Wise.

Travelers (minimum age, 18) pay a $30 annual fee to participate in the program. They must submit personal references and be interviewed by a Servas representative in their region. Servas then provides a list of host families, and it is up to travelers to make their own reservations.

Frequently host families are travelers themselves, and they will make an effort to show visitors around. The bulk of travelers from the United States tend to be of college age or senior citizens.

For information: US Servas, 11 John St., Room 706, New York, N.Y. 10038, (212) 267-0252.

* INNter Lodging Co-op: As the name suggests, INNter Lodging operates on a cooperative basis. Families make a room available to guests, and in exchange they can stay at other homes throughout the United States for an established rate of $4 (shared bath) or $5 (private bath) per person for adults and 25 cents for children using sleeping bags.

"Thus, lodging costs are reduced to a fraction of what they are in hotels and motels," says founder Bob Ehrenheim, a high school teacher in Tacoma, Wash. The organization is in its fifth year.

Travelers pay an annual fee of $45 to obtain a list of about 400 participating households in the United States, and they must agree to make a room in their home available for at least a three-month period. Guests are accepted only at the host's convenience.

Unlike Servas, guests are not expected to join in family life or help with chores. Stays can be as short as one night or as long as two weeks, if the hosts agree. One 22-year-old British student, visiting this country for six months, has stayed in 40 homes, says Margaret Ehrenheim, wife of the founder and a teacher herself.

Host homes tend to be clustered on both coasts. Because these residences are located outside city centers, guests generally need a car.

For information: INNter Lodging Co-op, P.O. Box 7044, Tacoma, Wash. 98407-0044, (206) 756-0343.

* Youth Hostels: For more than 50 years, American Youth Hostels has been providing inexpensive lodging to adventurous souls. There are now 275 hostels in 40 states and 5,000 more in 61 other countries. Despite the name, they are open to travelers of all ages.

Single travelers abroad traditionally have found hostels an excellent place to make new friends, if only for a day or two.

With some exceptions, a bunk for a night should be no more than $8 in the United States, although there is an annual membership fee of $20 per adult (ages 18 to 59); $10 for 17 and under or 60 and over; or $30 for a family.

Like the hostel at the Grand Canyon, most are located in prime tourist destinations, including cities and national parklands. One of the newest is in Las Vegas, within walking distance of casinos and nightclubs. In some countries -- Scotland is an example -- hikers can trek from hostel to hostel in a strong day's walk.

Accommodations vary with the hostels, but generally there are separate sleeping quarters and bathrooms for men and women. Most hostels have kitchen facilities so guests can prepare their own meals, another cost cutter.

The annual "AYH Handbook," free to members, lists accommodations in the United States and abroad. Reservations often are necessary. Hostels generally are closed from 10 a.m. to 5 p.m. -- so you can't sleep in or return to your bunk for an afternoon's nap -- and there may be a late evening curfew.

For information: American Youth Hostels, Membership Services, National Administrative Offices, 1332 I St. NW, Suite 800, Washington, D.C. 20005, (202) 783-6161.