Do you like souvenirs? I would recommend that you hold off on buying any until your last day
if there is any money left over. Your budget is doable if you are not constant snackers and bring a water bottle. I should think a burger type lunch (we like Richters, but I've read others that didn't - YMMV) and a nice dinner with not too many drinks ought to do. We don't do breakfasts. Maybe you can get by with a bagel or a pastry. We had to have the butterbeer at IoA but we didn't feel compelled to get the special mug.
A lot of the counter service restaurants have those huge soda cups (they offer Minute Maid lemonade and that sort of thing) which are not refillable, if memory serves, but they are really large, and the drink selection is good, so I should think that it would wet your whistle. We only needed one of those each at lunch and a large bottle of water each during the day but it wasn't overly hot when we went. Keep hydrated or you'll get sick. Don't skimp on fluids! There are tons of drink choices in both parks, so reserve some of your budget accordingly.
Don't forget the photographers that are positioned in the front of the parks. You can buy those pictures if you are so inclined. I don't know what they cost, but they must be fairly pricey. We bought the HPFJ ride pic which ran us upwards of $20 but I can't remember so don't quote me on that one.
I honestly couldn't tell you what we spent in souvenirs. We bought probably $150 worth, not including the photo (and that doesn't buy much). Bring a credit card with you, make drinks a priority in your budget and see what you end up with before you purchase souvenirs. I believe your budget is definitely do-able if you can maintain self-discipline. In reality, you could possibly get by on less $$ if you were really strict with yourself. I don't think I could do it, though.