The Caribbean for '$399*'! But what does the asterisk mean?

You dust the snow off your morning newspaper and there's a picture of a beach. With palm trees. And big letters: ''Caribbean Vacation - 4 days, 3 nights , $399*. Includes air fare and hotel.''

Teeth chattering and furnace kicking on, you say to yourself air fare and hotel? Your spouse checks the savings account: just enough for two - $800. This might just be possible. But wait. . . . Will you get stuck in the back of some flea-bitten hotel miles from a decent beach? What does that little asterisk mean? What's this in small print: ''Prices start as low as . . .''?

I recently flew to St. Maarten for four days to see if these super-reduced Caribbean vacations left me feeling as low as the price. Mine was an American Airlines package deal. Before I was through, I'd examined the major hotels that take part in Thompson's Vacations, out of Chicago; Travel Impressions, GWV, and Transnational Travel, out of New York, Boston, and Philadelphia; Carefree David, out of New York and Miami; and others.

Now, with the Caribbean resorts at the height of their winter season, the ads are ubiquitous in large metropolitan dailies such as the Chicago Tribune, Miami Herald, and Boston Globe, and in national magazines. There are also brochures and posters at travel agencies everywhere.

The small, half-Dutch, half-French island of St. Maarten seemed to offer the widest variety of Caribbean packages to compare. Tour officials say St. Maarten is having the best season in years, 5 to 7 percent above last year. (Also, reservations to St. Maarten were wide open. The week I wanted to go, my travel agent informed me that all hotels in the same package to Aruba, Barbados, and Antigua were filled.)

My four days were spent interviewing tourists, managers in the key hotels that participate in package deals - those including air and hotel for a given number of nights at reduced prices - and everyone from shop owners to taxi drivers and tour bureau officials.

The good news is that among guests in small, medium, and large hotels I found almost no one who felt they were misled, ripped off, or unpleasantly surprised. And I found almost no one who even knew of anyone who felt he'd been pitched a curveball - hidden or added costs, bad room, atmosphere, or service. None had the impression that the ad copywriter had been to another island. (The minor exceptions were people who thought their hotel was too large or too far from activities in which they wanted to take part - or too close to ones they wanted to avoid.)

Nearly all the tourists were American couples - either newlyweds or retirees - who wanted to swim, sun, and relax. I saw none accompanied by children. Their opinions of package deals ranged from ''couldn't be happier,'' to ''delighted,'' ''wonderful,'' and ''exceptional.'' Many of those who weren't on low-fare packages wished they had known about them. Some were dismayed to find they could have saved 60 to 70 percent of the price they paid for the same services.

That said, there are a number of things the tour-package hunter needs to be aware of:

* Price. Before I even got off the ground my fare was higher than the ''$399 *'' lead price: Traveling alone (the * means double occupancy) upped my package price to $520. And since my package required that I leave from New York's Kennedy Airport (instead of my local airport in Boston), there was an additional

Depending on which hotel you stay in (brochures picture the various hotels, and travel agents have books that describe each), the price could be much more. That's a second reason for the asterisk. Still another reason: If your package includes a Saturday or Sunday, the price will be higher. To keep my price down I was told I would have to leave on a Monday or Tuesday.

I wound up in the second-largest hotel on the island with a nice double room. However . . . my view was out the back - toward a dusty road and dirt parking lot beneath power lines. The price to switch to the seaside? $42 more per night.

* Hotel. The top five hotels offered in most St. Maarten packages are large. The smallest, St. Maarten Beach Club and Casino, has 75 rooms. Mine, the Great Bay Hotel, has 225 rooms, and the participating Sheraton Mullet Bay Resort is the largest in the Caribbean, with 580 rooms. So if a small hideaway is your idea of ''getting away from it all,'' you won't find it on a package deal.

The reason is that tour operators and wholesalers approach the larger hotels and purchase blocks of rooms. They guarantee occupancy and the hotelier gives them a lower price. This just isn't possible at smaller hotels.

Lida Huckleman, who runs Huckleman's Hideaway in the small town of Grand Case , says the tour operators never even approach hoteliers on the less developed French side of the island.

''The tourist agencies don't acquaint the tourists with the French side,'' she says. ''Once tourists are set up in a big hotel near Philipsburg (capital of the Dutch side), they come over here and see less development and cheaper prices - and fewer tourists. Then they want to come back.''

An attendant at the 25-room Pasanggrahan (my favorite, by the way, among the lodgings I looked in on) told me it does not need to contact tour operators to fill its rooms. Most of the clientele returns year after year and makes reservations a full season ahead.

Huckleman's Hideaway, like the Grand Case Hotel across the road, has efficiency apartments with sink and stove - unlike the hotels on the Dutch side. This is another feature you won't find in a package deal. Many island-goers love these apartments because they can save money by cooking for themselves instead of patronizing expensive restaurants for every meal.

A single at Huckleman's goes for $35, ($45 for double) - less than half the price of a room at most of the large hotels. There are only eight ''apartments, '' Ms. Huckleman says.

''But even at the height of the season, I'd say a tourist has a 50-50 chance of finding one,'' she says.

* ''Straitjacketing.'' Both the strength and weakness of the package deal is that it is usually very specific about what it includes. If the package is part of a charter deal (and therefore doesn't use regularly scheduled airline flights), departure and return flight times are occasionally changed (usually by hours, sometimes by days) by the charter operator. And there's sometimes no option to stay longer or leave earlier after purchasing tickets (ask your travel agent).

* Duration: I found one person who complained that a 7-days/6-nights package deal really ends up being only five days owing to travel time (4 hours from New York - more from elsewhere) both coming and going. Consider the number of days you want to rest or use for recreation - and you may realize that a short package deal like mine may not give you enough time.

Many people said they used the package deal only as a reconnaissance mission. They scouted out the type of hotel they wanted and made their own reservations for the succeeding year.

My four-day ''$399*'' vacation cost me $715. That included one $40 day trip to the island of Saba and an average of $30 a day for food. And I didn't spend anything on such common items as souvenirs, cabs, or car rental. It may have been higher than the tantalizing ad suggested, but for a first-time traveler, with no knowledge of the island, it was still better than any package I could have put together on my own.

The same trip without a package price (round trip from Boston - $518, plus $ 15 transfers to and from airport) in the same room ($90 per night) would have cost $963. And even staying in Lida Huckleman's Hideaway would have cost $798.

For general information and details about low-cost guesthouses, write to St. Maarten/Saba/St. Eustatius Tourist Bureau, 25 West 39th Street New York, N.Y. 10018.