No matter which resort you choose, and no matter which exchange company you work with, it is important to know exactly what you are buying. There are different kinds of properties, different kinds of exchange systems, different kinds of fees, and so on and so forth. Your best bet is to go into any sales presentation with a sound understanding of timeshare basics, so that you will be able to focus on differences in the “small type” when it comes time to decide which unit you want to purchase.

In its simplest form, timeshare is exactly what its name implies: a sharing of time at a vacation resort. You are buying the right to use anything from a studio apartment to a three-bedroom villa for a given period of time each year, when other timeshare owners are not using it. Remember: This is not the same as buying the whole resort and watching it appreciate in value like a wholly owned waterfront home. This is a less-expensive alternative to making a massive real-estate purchase, one whose primary selling factor is that it is supposed to be a hedge against vacation inflation.

What does that mean? Basically, the sales pitch is that if you buy into a timeshare program, you will — over the course of ten or twenty years — end up spending less on your vacation accommodations than you otherwise would have if you had plunked down cash for hotel rooms during the same period of time.

The Marriott Vacation Club Web site offers an electronic calculator to help you understand this principle. It lets you plug in the amount of money you typically spend on hotel rooms during the course of a weeklong vacation, then adds 5 percent inflation to that number for each of the next ten, twenty, and thirty years. The resulting number — the amount you are estimated to be facing for hotel-room payments during the next three decades' worth of vacations — is what the company wants you to use for comparison when considering purchasing one of its timeshare units.

For instance, according to the calculator, if you spend $150 per night on hotel rooms this year, you can anticipate spending $251.32per night in ten years, $409.37 per night in twenty years, and $666.81per night in thirty years (based on a 5 percent per year inflation model). By taking one week's worth of vacation each year for the next thirty years, at those rates, the calculator shows that you will have spent $75,341.65 on hotel rooms. Some other examples from the Marriott calculator:

• If you currently spend $250 per night on hotel rooms, your thirty-year total for one week a year of vacation lodging will be $125,569.42

• If you currently spend $350 per night on hotel rooms, your thirty-year total for one week a year of vacation lodging will be $175,797.19.

• If you currently spend $450 per night on hotel rooms, your thirty-year total for one week of vacation lodging will be $226,024.96.

Those are certainly some big numbers, and they're exactly the kind of prices Marriott and other timeshare companies want you to consider when deciding whether to purchase a $25,000 or $50,000 timeshare unit. It may seem like a lot of money up front, but when you compare it with the numbers for those hotel outlays, timeshare suddenly looks like a much better deal. And, when you compare the amenities of a private villa against those of a typical hotel room, the deal looks even sweeter.