A winter Caribbean vacation for two on $25 a day? Is there anything worth having for $25 these days when major resort hotels charge $100-$150 and up for a double?

Suprising there is, here is Puerto Rico. If you are willing to put up with some inconvenience, you can enjoy the same glorious sun, the same soft sand beaches, and the same greenblue water as the high rollers - and pay one quarter to one sixth as much.

I am not referring to Puerto Rico's paradores or inns. They are good buys, but they cost $20-$32 double just for the room. I'm talking about $25 a day for everything except the air fare to get here and a rental car, if you decide to hire one.

You do it by staying at Puerto Rico's equivalent of our national parks: the government-subsidized centros vacacionales (vacation centers), of which there are only three. Simple accomodations for up to six people, without food, cost $12-$15 a day at these centers. Not per person; $12-$15 total (family groups only). They are little known in the continental United States, because, since they compete with tourist hotels, they are forbidden to advertise. And space is limited.

The accommodations in each case consists of basic, Prefabricated cabins. Each cabin has two bedrooms, one with a double bed, the other with two double-deckers, thus sleeping six; a kitchen-living area, with electric stove and refrigerator, and a bathroom with shower. Most cabins have air conditioning and many have hot water. You bring your own linens, blankets and kitchenware - or rent them, when available, for a nominal charge. (Don't count on them always being available.) Food is for sale at a grocery center and at a short order lunch counter.

Provided you don't expect room service and night life, it's not bad. It's particularly good deal for a large family. The kids are freer, and food costs little more than at home. The family cook doesn't have a complete vacation, but you can't have everything. Besides, if you rent a car, there are good Puerto Rican seafood restaurants which are not expensive (though eating out will up your food costs). And there's still the air fare, of course.

For my money, the centro here at Boqueron, on the sunny southwest corner of the island, is the nicest of the three. The beach is a two-mile crescent of soft white sand lined with thousands of coconut palms. The water is tranquil, warm and the typically green-blue color of the West Indies. At nightfall the palm trees are silhouetted against a flaming sky just outside your front door.

A second, and newer, centro opened in 1977 at Punta Santiago, near Humacao, on the east coast. There, in addition to a beach (less attractive) there is also a swimming pool and two good tennis courts, facilities which are not available at Boqueron. The cabins are imaginatively designed, some having second-story breezeways where you can serve an outdoor meal. There is hot water, which you won't have at Boqueron, but no linens or dishes are offered for rent at Punta Santiago at any time, and it is more crowded there.

The third centro is 2,500 feet above sea level in the west-central mountains near Maricao, close to the peak of Monte del Estado. It is a lush, green and cool mountain retreat. Opened in 1970, it has a pool, a fireplace in each cabin, individual barbecue pits, and for a few cabins a magnificent view of the southwest coast of Puerto Rico, some 15 miles distant. A Scandinavian ambassador was using one the day I was there.

At Boqueron, 158 cabins are available, some for one family and others duplexes; at Punta Santiago, 72 units; at Maricao, 24 units.

You are allowed to reserve space at each centro for up to seven days, making application no earlier than 120 days in advance. You may visit any or all of them once every three months.

Write for an application form to Director, Centro Vacacionale, Boqueron, Perto Rico 00622, Director, Centro Vacacionale Maricao, Perto Rico 00706; or Director, Centro Vacacionale, Punta Santiago, Humacaco, Puerto Rico 00661. Sometimes when there are vacancies, the directors at Boqueron and Maricao (not Punta Santiago) will let you prolong your stay beyond one week, or come back sooner than three months.

Astonishingly, the centros are often virtually empty in winter, except on weekends and during holiday periods (such as Christmas through Jan. 6). Puerto Ricans prefer to take their regular vacations in summer, and use the centros as weekend cottages the rest of the time. So from November to April, except from Friday at 3 p.m. to Sunday at 3 p.m., you may not even need reservations. You probably can just drive up and rent a cabin - and in fact, have the whole place almost to yourself.On Friday and Saturday nights, if necessary, you can switch to a nearby motel or inn. But it would not be wise to plan a vacation at a centro and fly to Puerto Rico without a reservation.

Getting around the island requires planning. A rental car costs at least $20.95 a day or $145 a week (plus gas in each case; unlimited mileage). But if you have spirite of adventure, and preferably if you speack soome Spanish, you can use publicos, which are regularly scheduled, share-a-ride taxis.

There are thousands of them on the island - autos identifiable by their yellow license plates. They operate on bus-like schedules which are posted at the airport and which are also available from the Puerto Rico Tourism Companyy, GPO Box BN, San Juan, Puerto Rico 00936. They cost only about 6 cents a mile, per person, or about $6 from one end of the inland to the other. But the centros are not regular publico routes, so you would change buses, and then negotiate a special price with the final driver for door-to-door service. This door-to-door service will probably cost you about 24 cents a mile, since you will in effect be hiring four seats - the whole sedan.

Some miscellaneous suggestions:

It is well to bring a couple of 100-watt light bulbs and even a fold-up desk lamp if you expect to do any reading at night at the centros. There is electricity (110 volts). but the cabins are dimly lit. Portable radios and TV sets are allowed, if you bring them with you - the programs, however, are mostly in Spanish.

The climate in Puerto Rico's mountains is very different from the seashore, and you will need to dress accordingly. Figure on winter temperatures from 70 to 90 at sea level, and from 55 to 75 in the mountains. You'll be glad of the fireplace at night at Maricao, and you'll want a sweater.