I'm vegetarian - here are my favorite value meals, priced around $6-7:
MK:
A side of rice and a side of beans at Pecos Bill. Put them on a plate and add toppings from the side bar.
A side of broccoli and a side of couscous at Columbia Harbor House.
A side of mashed potatoes and a side of green beans at Cosmic Rays.
The nutella and fresh fruit waffle at Sleepy Hollow
AK:
Edamame from the Caravan cart in Asia with a side of fresh fruit (e.g. watermelon, strawberries, or pineapple) from the Harambe Fruit Market. For a heartier appetite, swap one of these for falafel or samosa from Mr. Kamal's cart in Asia.
EP:
World Showcase has tons of options! Tangierine cafe and La Cantina de San Angel are my favorites, although they are about double in price (~$12) which is still a value relative to the amount of food (two can easily share a vegetarian platter from Tangierine - La Cantina, maybe, maybe not depending on appetites).
HS:
Blech! The only thing I like there is HBD, but it's pricey (and not IMO not worth it for the vegetarian options). If I can't make it over to EP to eat and don't want to spend $$$ then I have a smaller version of the Cobb salad (~$9) at the outdoor HBD lounge.
For buffets, I think Tusker House or Boma offer the best value in terms of quality and selection (relative to other buffets on property).