FACT: THE CARIBBEAN is expensive for a vacation, especially in winter and particularly in the French islands where the French penchant for style and good food drive the Caribbean's normally high prices into the stratosphere.

True, usually, but it doesn't have to be that way, thanks to loosened charter flight regulations authorized by the Civil Aeronautics Board in the last couple of years.

It is not all uncommon for a charter flight with hotel accommodations to cost half the price of regularly scheduled airline service plus comparable hotel reservations. In addition the charter flight invariable will offer more services, including free transfer to and from the airport, and usually get you there faster with less stops -- particularly in the case of the furtherout, non-American islands in the Caribbean.

The Constant Companion and I recently spent a week in Martinique, generally considered one of the highest-priced of the islands, for slightly more than $1,000 for everything -- flight, hotel, meals, car rental, sailing, snorkeling equipment, tennis, even a few small gift purchases.

The trip easily could have been done for $200 to $250 less, depending on choice of hotel, meals and whether you want to rent a car for a couple of days or just take a few steps from the hotel each day and collapse on the beach.

There is really only one simple key to the whole money-saving business -- find a cheap charter. A look at the Sunday newspaper travel sections is usually as far as you need to go. A friendly travel agent can often steer you to a cheaper charter than you had even planned, as was the case for our trip.

The basic price of the trip to Martinique was $300 per person for roundtrip transportation and a double room. We added another $78 by getting accommodations in the island's poshest hotel, with the hotel listing the room with balcony overlooking the sea at $115 per night. No, that does not incoude meals and, yes, the room was very nice, but you would have to be crazy to pay that much for it.

This particular charter package was especially cheap, we were told, because the bad weather in Chicago had forced the charter company, Trans National Travel of Boston, to move the trip to Philadelphia and Boston on short notice. Trans National charters the planes from Capitol International Airways and reserves hotel rooms a year in advance to get the best rates.

The way TNT plans it, every Sunday from mid-December to mid-May, a stretched DC-8 with 250 passengers flies from somewhere on the East Coast or the Midwest to Martinique and brings back last week's 250 passengers. No matter whether this week's seats are sold or not, howe er, a plane has to go down to get last week's passengers.

That's the kind of situation where an alert traveler can make a deal, especially if you are able to go on two or three weeks' notice.

The savings can be considerable -- during the current high season the regular charge for charter flights to Martinique runs about $470 to $500. Even that, however, is a significant saving over the regularly scheduled flights which cost $455 for an open ticket or $368 for a weekend 3-21 day excursion, with hotel costs still to be added.

So, the moral is to buddy up to a travel agent and let the company know you are interested in any great deals that come along.

Yes, there are a couple of small drawbacks. Our flight left from Philadelphia at 5 a.m. and made a stop in Boston before arriving in Martinique at 12:30 p.m.

Philadelphia International Airport is about three times as long a drive from downtown Washington as Dulles or BWI, but the extra four hours of roundtrip driving works out to be a savings of about $50 an hour. Besides, you can soak up all that history and good food in Philadelphia and, in effect, staying the night at the airport motel won't even cost you anything. The motel allows customers to leave cars there free while away on a trip, and even tosses in free transportation to and from the airport. The price of the room about matches the airport parking fee for a week.

True, 5 a.m. is an unspeakable hour to have to catch a plane, but regularly scheduled flights to Martinique are much more time-consuming. Eastern requires a plane change in Miami and then stops in Puerto Rico and St. Thomas; American calls for a flight to New York and an overnight stay before getting its morning nonstop to Martinique from JFK Airport.

The rest of the money-saving is up to you since it involves ways of getting around the high cost of restaurants in Martinique. There is really no way you will escape dinner with wine for two for less than $20 and twice that is not at all uncommon. The big hotel restaurants feature groaning buffet tables for about $35 for two, excluding wine, but one meal like that will be about all you can eat in a day. And everywhere you go you will find food that can't be duplicated in Washington at any price.

The small markets provide an abundance of bread, cheese, croissants and yogurt, which can easily take care of other meals if you wish.

Or you can be lucky like the Constant Companion and find a couple of gigantic avocados on the ground at a banana plantation and thus provide the basis for a couple of lunches on the balcony overlooking the sea.