(FORTUNE Small Business) – Never trust a thin chef. These words come from a cousin of mine, who is a chef, and, yes, is a little heavyset. The Web version of this is the free Web service. If it's so good, why isn't it charging?

Free offers on the Web have appeared to be a boon for small business. Why should you pay for building an e-commerce site when someone will give you one for nothing? Why should you buy a fax machine or an answering service when there are Websites performing the same functions gratis? Millions have signed up to use various Net freebies following this logic, but doing so no longer makes sense.

In the current market environment, Web services are realizing that free is, in fact, one of those four-letter words. EVoice, for one, is a Web-based voice-mail service that has more features than the phone company's comparable call-answering product. When it launched last May, eVoice cost nothing; the phone company charges $10 a month. How long did anyone think that discrepancy was going to last? As it turns out, not long at all. EVoice recently started to charge--$9.95 for installation and $1.95 a month for basic service, $5.95 for premium. While I guess I should be happy that eVoice finally realized its service is worth something, this move smacks of 11th-hour desperation, especially when you consider how loudly management touted being free. The chickens are not only coming home to roost, but they're also rooting through the sofa cushions for loose change.

You also quickly become a second-tier customer with the freebies. When the e-commerce site builder FreeMerchant started to offer for-pay services in addition to the free ones on its site, one of the features that they began charging for is--you guessed it--customer support. Hey, the logic goes, that stuff costs money, and you're not giving us any.

Although you may have saved a few bucks in the short term by using these sites and their still-free brethren, what happens when they go out of business? If you were relying on AltaVista's free Net access, which shut down in December, you're now incurring costs to sign up with a new Internet service provider. More important, how much time will you spend setting up anew and telling everyone you have ever contacted through AltaVista that you have a new address? Now imagine how much you'll spend rebuilding your Website with another service. Those switching costs cannot be underestimated.

Save yourself the agony, and use only those sites that know they have something of value and charge for it from the outset. Most sites that pair up buyers and sellers, like eBay, have always charged sellers to post items for auction or sale, knowing that its delivery of an interested audience of millions to your products' saleis worth something. McAfee.com costs $29.95 for an annual subscription to its online virus scanning and PC tune-up service, something you've been happily buying in a box for years.

The siren song of free services is alluring, but the risk is significant unless you know it's something being subsidized by a big company, like Palm's free Web personal information manager, MyPalm. Ultimately, ask yourself this: Which would you rather patronize? The all-you-can-eat joint with the dirty shrimp, or the fairly priced neighborhood bistro that grows its own vegetables?

Don't believe the hype? E-mail david_lidsky@timeinc.com.