EXACTLY, if the value was exactly equal to the price I wouldn't buy it. Why give up a discount on tickets and get stricter payment and cancellation policies?
Disney says the dining plan gives guest a 40% savings on their food. I think that may be a little high.
Boma is worth around $32 with tax and tip, a CS meal is worth around $10 and a snack $3-$4 That totals around $45. That number would be higher if you assume a guest has TS meals in restaurants such as Le Cellier and Chefs de France or counter service meals at places like Pepper Market. The dining plan is popular, in part, because guests think they're getting, and some cases do get, a fantastic deal.
My point is guests should be using the value of the food they think they'll be getting in doing the analysis.