I am not sure if there is a "cheapest" option like you are looking for. Pricing is usually consistent on property. However, there are some CS that offer options that are more cost effective. Like Flame Tree at AK offers ribs and chicken meals that are a little pricier but offer much more food. El Pirato et Perico in MK has good tacos and burritos that are less expensive than the burgers, etc at Pecos Bills. Epcot offers some good CS options that vary in price, but also vary in options.
I guess the best response to your question is to check out the menus here or on Allears.net so you can determine what you would like to eat.