I try to allow $75-$100 per day,tend to buy tee shirts and travel DVDs,but,expect to have some dollars left over as in the words of Arnie,"I'll be back" so any excess goes to the next trip.
It really depends on the type of meals you want,if you are in a city,there will be quite a range of restaurants,from inexpensive to very expensive.If you are in a smaller town,or,place there may not be much choice.Bear in mind that US portions are usually large,do not be afraid to ask for a doggie bag,if you can only manage part of the meal,particularly sandwiches.