If you're a regular reader of SmarterTravel, you'll notice frequent offers for round-trip fares as low as $58, $18, or even $2. These amazingly low prices usually come from Spirit Airlines, a Florida-based "ultra-low-cost carrier." The best prices are available only to members of Spirit's $9 Fare Club.

The $9 Fare Club entitles its members to members-only fares. In addition, travelers flying on the same ticket will receive member prices, too, which means you can effectively split a membership with your spouse, family members, or friends if you travel together. The membership also includes private offers on other travel services such as hotels and car rentals.

But, is the cost of membership worth the price? The annual membership actually costs $59.95, or about $4.99 per month, and a 60-day trial membership costs just $9 (and automatically renews into an annual membership at the end of the trial period). You can also obtain a free membership to the $9 Fare Club by getting a Free Spirit MasterCard.

If you frequently fly between the cities Spirit serves (a heavy emphasis on Florida, the Caribbean, and Latin America), membership is worth the cost. If, for example, you can snag a $2 fare, even with the membership cost and taxes and fees, you'll still be getting a bargain compared to other airlines' prices. But, you must keep in mind that availability is often very limited. You'll find that fares are only offered on about a dozen select dates, and even then, you must rush to book early or risk the best deals selling out.