Vegetarian and low-calorie diet programs are usually, but not always, the cheapest. The cost of your diet program depends on the level of support and guidance offered and whether or not the program includes meals and snacks. For example, you can start WW for $3 per week, but you have to do your own grocery shopping. On the lower-cost end of programs that provide food, Nutrisystem provides meal delivery and support for around $10 per day. On the higher end, the Zone Diet costs about $50 per day for meal delivery.