Food portions at Disney tend to be LARGE. You may want to share meals. Not only does it save money, but then you don't have to carry around left overs all day.

This is especially true if you eat at any sit down places.
At MK, they have a family rotisserie chicken meal for around $14 at Cosmic Ray Cafe.
Kids meals are usually pretty cheap. Once in awhile I would even order a kids meal. I like their chicken tenders and it was just the right amount of food. I think you get 2 chicken planks (think 2 big chicken strips, not nuggets) and fries and a drink with the kids meal. Check out
http://www.allearsnet.com for menus with prices.
We bought one bottle of water and then just refilled it from the water fountains. Bottled water costs $2.50 at WDW. We think the water in Florida is fine but some say it "tastes funny." The tap water works for us! But, if you are staying at a condo, maybe you could bring some bottled water with you.
If you want to do a character meal but be on a budget, try the Ice Cream social at Garden Grill in EPCOT. IT is around $8 each for adults and kids. You get a big ice cream sundae thing and get to see the characters too! Its cheaper than the other character meals but fun! Just order water, though the drinks can jack up the price.
At Animal Kingdom they have a meal deal. You order a meal at a counter service place and it comes with tickets for a free water/pop and ice cream/popcorn for later. Ask for the meal deal when you order. I had read about it on these boards, so I knew about it when we went last time. Good thing because as I recall it wasn't advertised much at the restaurant.
I think having fun has A LOT to do with attitude! We go on a budget often (at least compared to some of the amounts I see spent on this board!!) but have a great time because we LOVE Disney and are happy to be there. You will have JUST as much fun as your friends with the bigger budget so ENJOY!
Maggie