Diet programs that require customers to buy pre-packaged meals cost around six times more than those that simply guide dieters on what to eat but allow them to buy their own food.

Typical costs:

Weight Watchers, which does not require pre-packaged food, is the highest-rated overall program, according to customer reviews compiled by ConsumerSearch. Weights Watchers charges around $47 per month for membership. The company encourages customers to visit local meetings, saying dieters who attend regular meetings have three times more success than those who follow the diet with online support only.
A membership to the diet clearinghouse Ediets is only $5 month, but costs go up if dieters choose a program that involves buying food. Ediets offers its own DeliciouslyYours meal plan for $16 to $26 per day, which is similar to what Jenny Craig charges.
Slim-Fast, a popular supplement-based diet using shakes and snack bars to replace some meals, provides a free personalized nutritional program when customers order products directly online. Slim-Fast granola bars cost around $6.50 for a pack of five and four shakes cost $8.49. Customers using a snack bar or shake to replace just one meal a day every week could easily spend $60 a month on supplements.
Jenny Craig and NutriSystem, two of the most popular programs that provide meals, cost between $300 and $350 per month for food. Jenny Craig also charges an initiation fee of $300 and a monthly program fee of around $18 for a three-month plan. If dieters sign up for longer terms, they often receive discounts on food and as much as half of their initiation fee back if they come close to reaching their diet goal.
NutriSystem charges around the same as Jenny Craig for meals, but does not assess a membership or start-up fee. It offers several different programs designed especially for men, women, vegetarians and diabetics.

What should be included:

For programs that require dieters to buy pre-packaged food, there should be three entrees per day, plus snacks and dessert delivered to your home once a month.
Most programs select the first month's menu to give dieters a full sampling of their selections, but after the first month, dieters get to customize their meal plans. There should be a variety of entrees, including vegetarian items and those designed for low-sugar intake.
Most diet programs offer free advice online or over the phone for members, and many host online support forums.

Discounts:

The least expensive option for dieters is to read online for free about self-guiding plans like Atkins and the South Beach Diet. While these companies sell food products online, dieters can follow the basic principles of these plans without buying the food.

Shopping for diet programs:

ConsumerSearch compares diet programs based on customer reviews. BestDietForMe.com offers reviews of 60 top diets.
Ediets hooks dieters up directly with numerous well known-programs as well as more obscure diets like the Mayo Clinic Plan designed by doctors, or its own alternative to Jenny Craig.
To find a local Jenny Craig center, check out the company's online directory. Jenny Craig customers can choose between two kinds of programs, which include working with diet assistants at local Jenny Craig centers or working privately from home with online and phone support.