Until recently, the general rule of thumb for most cruise fares was that you could expect to pay an average per diem of $200 per person, double occupancy. But with the cost of some cruises being cut by as much as 50 percent, cruising has definitely become a buyer's market. Here is a sampling of some of the better fall and winter bargains:

*On Princess Cruises' 14-day trans-canal cruises sailing from now until mid-December, $800 will be knocked off the price for any couple. The Panama Canal voyage normally costs between $2,966 and $4,242 per person. Next winter, a savings of up to $1,200 per couple will be available on almost all of the 14-day Panama Canal cruises, including sailings of the new Royal Princess. Princess is also offering discounts of $500 per couple on seven-night cruises to the Mexican Riviera and to the Caribbean. Prices for the cruises without a discount range between $1,456 and $2,009 to Mexico, and $1,428 and $2,009 to the Caribbean.

*Carnival Cruise Lines offers free air fare or a $100 allowance on all its cruises. Three ships depart from Miami every week. On Saturday, one ship sails to Nassau, San Juan and St. Thomas; on Sunday, another vessel goes to Cozumel, Grand Cayman and Ocho Rios, and each Friday the third ship goes to Nassau and Freeport for four days and leaves again on Monday for a three-day cruise to Nassau.

On the West Coast, a fourth Carnival ship leaves Los Angeles for a seven-day cruise to Mazatlan and Puerto Vallarta. A seven-day Carnival cruise to the Bahamas, the Caribbean or Mexico costs between $895 and $1,795.

*Pearl Cruise Lines is offering inexpensive air fare until the end of the year on its 14-day cruises to Asia. From San Francisco, Los Angeles, San Diego, Seattle, Portland and Phoenix, you can fly round trip to the cruise embarkation points, including Hong Kong and Singapore, for an incredible $99. From New York, Miami, Chicago, Dallas, Fort Worth and Houston, the cost is just $289 for a round-trip ticket.

*Norwegian Caribbean Lines is offering a Sea Saver fare between Sept. 1 and Dec. 16. A seven-night cruise to the Caribbean from Miami costs only $599 per person for an inside cabin and $649 per person for an outside cabin. You can book the cruise only four weeks in advance and can select only the date -- Norwegian Caribbean will select the ship and specific cabin. The Sea Saver fare represents a savings of at least 30 percent. Regular cruise prices for the same trip range between $975 and $2,710.

*Eastern Cruise Lines' Emerald Seas has three- and four-night cruises out of Miami to Nassau and Freeport, as well as Little Stirrup Cay, a private Bahamian island leased by the company. Cruising for three nights costs $290 and four nights $385.

*Sun Line Cruises has a special two-week "Gems of the Caribbean" Orinoco river cruise, leaving Jan. 4, 1985 from Fort Lauderdale, that will be discounted by $200 per person. The regular fares are $2,545 to $4,785. On two positioning cruises, a 26-day voyage leaving Athens Nov. 25 and going to Fort Lauderdale, couples will receive a $500 discount. Regular fares are $3,350 to $6,590. The discount also applies to the March 10 cruise returning the ship from Fort Lauderdale to Athens. That trip costs between $3,170 and $6,070 without the discount.

*Epirotiki Cruise Lines will fly you free from New York to Greece to board its ship for a westbound transatlantic voyage and Caribbean cruise, and charges only $100 for the return flight from Barbados to Miami or New York. For the eastbound transatlantic cruise, it will provide free one-way air transportation from Athens to New York, and you pay $100 for the flight from New York or Miami to Barbados, where you meet the ship. The cost for the cruise ranges between $2,600 and $3,800.

*Royal Viking Line has a special "Two-Step" offer this winter on its one-week Nov. 3 sailing from Fort Lauderdale to the Caribbean. You get a certificate for one-half the cruise fare that you can use for a 10-day cruise in 1985. Prices range between $931 and $4,004.

For the holidays, RVL will offer discounts of $400 per person on its three cruises: an 18-day trans-canal cruise, a 24-day Mexico and Hawaii cruise, and a 21-day Hong Kong-Sydney cruise. An extra bonus applies to the Bali-Oceania-South Pacific cruise -- passengers get $400 off and fly free.

*If you sail with Cunard on the QE2 from New York to the Caribbean and the Bahamas, the second person in the cabin pays less than minimum fare on all five- to 14-day cruises. For example, a 10-day cruise to the Caribbean normally costs between $1,910 and $2,725. If you book the special rate, the second person pays only $1,745, regardless of the cabin type.

*Costa Cruises charges passengers leaving from New York or Miami on a seven-day cruise of the Carribean just $789 per person. Ships depart and return to San Juan every Saturday.

*Sitmar Cruise Lines will sail you for seven days to St. Thomas, San Juan and Nassau for $845 per person. If you leave from Fort Lauderdale, a discount of $100 per person applies. The line, which will offer free air fares on this itinerary in 1985, only charges minimum add-on airfares -- usually not exceeding $50 -- from most U.S. cities.