BankRate.com had this article on MSN Moneycentral:
The article runs down the costs associated with well-known diet programs like Jenny Craig, Nutrisystem, Weight Watchers, and The Zone Diet. These programs aren't cheap. The cheapest of the four for losing 30 pounds is under $100 (for online registration at Weight Watchers, not including food) to over $5,000 for daily food delivery in some cities for The Zone Diet.
A former member of our church went on one of these programs (his was $10 per day including food) and lost the weight initially, but then it came back.
I can see the plans being pretty difficult to stick to, and I can see it being pretty easy to cheat (eat food outside of what's allowed). And that's the killer cost.
I'm sure the added external structure of these programs is worth the money, but let's face it: It's not in the best interest of these companies to get you to a point where you don't need them. They make money by selling controlled portions of food and other services. And they also make money when you fail and come back.
So though these programs may be good at kick-starting a diet, the dieter should look for a way to maintain the eating habits without that company. Also, if one of these programs doesn't seem to be working or is just too painful to follow for whatever reason, get out. That's not failing; that's just being pragmatic. Try something else after figuring out why that particular plan didn't work. There's no real reason to pay a premium for food if it's not serving its intended purpose.