Let me start off by saying that I'm a big fan of Signature dining...I was raised by foodies, and on family trips, it's Signatures all the way. That being said, I'm 26 and getting ready to go back to grad school in the fall to be a nurse anesthetist, and most of my friends are still in grad school also...so I'm pretty well versed in budget trips to WDW.
*If you're staying at one of the values or mods, breakfast in the food court is awesome! It's very good, as well as convenient and cheap. I could eat breakfast pizza at POP every day for a whole trip

Another good option if you're not big breakfast eaters is to bring things with you like pop tarts and granola bars to keep in the room.
*I'm a big fan of counter service. There are some really great places out there. I really enjoy Pecos Bill's and Columbia Harbor House in MK. In Epcot, Sunshine Seasons, La Cantina de San Angel and Tangierine Cafe are all very good. I also really like the sushi and udon combo plates at Yakitori House. DHS has Starring Rolls (really good sandwiches and desserts). And I like both Flame Tree BBQ and Yak and Yeti CS at AK. Also, you should definitely check out Earl of Sandwich, Cookes of Dublin and Wolfgang Puck Express at Downtown Disney.
*My favorite budget TS places are Les Chefs de France (really only for lunch, the prix fixe menu is awesome), Sanaa, 50's Prime Time, Sci Fi, The Plaza, GF Cafe, Kona, Via Napoli is do-able if you share, House of Blues and Raglan Road. I find it's much cheaper to stay away from the buffets as I would never eat enough to justify the price of them.
Hope that helps! Have a great trip.