I have a few coupon books which I got on ebay with discounts etc for meals in Orlando, if you send me a PM I will post one to you (I got 3 of them from ebay!). They are good until December 2005.
Ponderosa advertises an all you can eat breakfast for $3.99. If you two can stomach a big breakfast, I would probably have something like that, maybe save a piece of fruit if you can for later and then skip lunch and just buy a hot dog or something in the park. Then for dinner try a family restaurant outside disney. The coupon books I have seem quite good, some give you up to 25% off the whole meal.
If your hotel has a fridge you can buy things like milk and juice at a Winn Dixie or Albertsons. You can also get mini cereal packs - I don't know if they still have them but when I was growing up in Canada you could buy these little cereal boxes with perforations in them, you just open them up and pour in the milk! Bags of salad and dressing, luncheon meats etc. Also, nutrigrain bars are great for snacks and are light and easy to keep. American grocery stores have lots of different kinds of 'bars' to eat. You can also get things for your room that don't need refrigeration like Jello pudding packs etc (just buy some plastic cutlery).
The other day I got this free 'cooler' bag at M&S which i'm bringing with me for beach days, I can put our lunches in there and hopefully they will stay cool. It takes up no room and folds up, something like that might be an option for your St. Pete break.
Also, if you're not too worried about your cholesterol (

) you could try some fast food places like Wendy's which do value items for about 99 cents I think and these include things like jacket potatoes, hamburgers, caesar salads, milkshakes etc. It's not ideal but you're on holiday so a couple of burgers won't kill you!
My husband and I are going in 4 weeks and although we're excited we don't want to spend too much either. We're hoping to keep our spending under $150 a day for 2 weeks (that's with clothes shopping etc). As we're staying in a villa we want to keep eating out to a bare minimum (once a day!).
Hope you guys have a great time
Eloise