Eat your sit-downs at lunch--it is a nice respite from the day--the meal is much less expensive, and you can just snack your way through dinner.
I find that with the sit downs--I am still full even at dinner time, so I can budget less on the meals that way.
If you stick with drinking water at the meals--perhaps limit your sodas to one a day or something...you can get away with a $20 sit down lunch, and $10 or less on counter service for breakfast and either lunch or dinner. In theory--you could get away with $50 per day. Remember your food weaknesses and budget accordingly--I always have to have pastry/sweet from The Boulangerie/Patisserie in France at EPCOT--nothing beats a French confection. An extra $5-$10 per expected indulgance would be good (be it a beer, glass of wine, or coffe + baked good, pretzel, whatever).
You can get creative--I saw elsewhere that you do not have to order the "meals" at counter service restaurants--you know the burger and fries meal. So you and your friend can maybe buy 1 meal and then just buy a 2nd sandwich.
I know at the all stars you can order a pizza--I assume this would be the same at Pop--you can split a pizza in the evening.
My breakfasts---I never ever order the big meals, my breakfasts are usually a grain item and a fruit--now these are pricey at WDW--but you can eat breakfast for less than $5 bucks as opposed to nearly $10 when you order a platter. If you are driving--you can always stop at Publix on the way (in my mind I know where it is, but unfortunately cannot recall the directions)--you can get some cereal bars, perhaps some boxed OJ (refridgerated kind) and put in the cooler and your own fruit--and you'll have breakfast daily for pennies of what it would cost.
As far as character meals--do them at breakfast (this is the only real breakfast meal I do eat--and we like Crystal Palace with Pooh) or for dinner---breakfast about $16, dinner about $26--before tip. Pricey, but for a single person a character dinner could be much cheaper than dinner at EPCOT for example.
Happy planning--I miss the days of college and going without kids--though I do love going with my kids
