If you were hoping to fit into that itsy bitsy, teenie weenie, yellow polka dot bikini by Memorial Day, you have about eight weeks to shed those extra pounds that have been hidden behind bulky sweaters since the holidays.

It's no secret that diet and exercise can get you there, but it sometimes helps to have a little more motivation. Some people pay for a gym membership to reinforce their determination to exercise, hoping that the dread of wasting that membership money will overcome the twin sins of sloth and gluttony. Others feel that a weight loss program will better keep them away from temptation.

Before one of those programs lures you with low introductory fees and monthly payments that sound downright reasonable, consider the results of a recent Bankrate calculation. They set out to find out how much it would cost to lose 30 pounds using five different weight loss programs. Here's what they discovered:

  • Jenny Craig offers three different memberships for three different durations. For those with fewer than 10 pounds to lose, membership costs $49. Otherwise, it's $199 for six months or $399 ($358 prepaid) for a year. Add to that the cost of food, at $11 to $17 per day. (By my calculation, if you sign up for the six-month program and buy four months of the lowest-cost food, you'll pay about $1,500 total.)
  • At L.A. Weight Loss, you can expect to pay a $5 to $7 weekly fee, in addition to an undisclosed setup fee. (One center quoted a fee of $174.) The cost of its special nutritional bars runs between $18 and $30 each week. Your initial cost if you participate online is $219, including a four-week supply of the bars. Anyone who starts the program hoping to lose 30 pounds will pay for 15 weeks of weight loss, six weeks of stabilization, and 52 weeks of maintenance, for a total of 73 weeks. Bankrate calculated that using the setup fee of $174 and $7 per week charge, the total cost of the plan is $685 for those 73 weeks, not including the cost of food and nutritional bars.
  • NutriSystem (NASDAQ:NTRI) works by selling prepackaged food to its dieters. Participants get the best deal if they sign up for automatic deliveries that only stop when canceled, which cost $293.72 each month (including shipping). You'll have to spring for your own fresh greens and dairy products. Assuming it takes you four months to lose your 30 pounds, the cost comes to about $1,175.
  • Weight Watchers (NYSE:WTW) registration costs between $15 and $20, and weekly meeting fees range from $9.99 to $13.99. (Register for the online program instead and you'll pay $46.90 for the first month and $16.95 for subsequent months.) If it takes 20 weeks to reach your goal, you'll spend between $215 and $300 (or $98 online). But groceries cost extra.
  • The Zone Diet, available in some cities, delivers food daily to participants. Sign up for a 30-day plan and you'll pay $39.99 per day. If you want to try it out for two weeks, you'll pay $44.99 per day. If it takes three to four months to lose your 30 pounds, you'll pay somewhere between $3,600 and $4,800.

After doing a quick review of the websites for each of these programs, it became quickly obvious to me that it's not at all obvious to the average dieter how much it costs to reach a weight loss goal. Needless to say, the programs devote much more space to motivational pitches than to details about prices for the program or food. Prices often get displayed as average weekly fees, with lots of asterisks leading to lots of fine print. In some cases, the prices weren't displayed at all.

Before you become convinced that a program will turn you into Pamela Anderson or David Hasselhoff, ask for all the details about a program's costs and its success rate. Take the time to add up the amount you'll pay during the duration you expect to be on the program. And if you want extra motivation to lose the weight, you might enjoy Robert Brokamp's admonitions against eating your retirement.

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Fool contributor Mary Dalrymple prefers to spend her dollars on ice cream instead of weight loss programs. She does not own stock in any company mentioned in this article. She welcomes your feedback. The Motley Fool has a disclosure policy.