Pure & Simple is indeed inside the Wonders of Life pavilion in Epcot's Future World. My father, the vegan, can even eat there!
Another good, inexpensive Epcot place is the Sunshine Food Fair inside the Land Pavilion, lower level. Many different stands for all different tastes, and inexpensive!
At MK I would stick with Pecos Bill's and CHH. Filling meals on a budget! Same for AK's Tusker House.
MGM I cannot find a decent inexpensive place to please myself, DH, and our 3 year old DS. This is one park we plan on a bigger meal at a sit-down dinner.
MulanMom mentioned the Trail's End Buffet at Fort Wilderness. This is one of our favorite places! It is so nice to take the boat over from MK, walk around the beautiful grounds, and have a meal here. Something for everyone at every meal! FW also has tons of inexpensive or free enteratinment! The petting farm and stables, pony rides, bike and canoe rentals, the free movie and sing-along at the campfire each night. Can you tell we spend a LOT of time at FW, even if we're not staying there?