Hi JustMargaret,
Budgetting is such a personal thing but to offer some guidance here are a few things to think about....
The good thing about America is that in terms of meals there is something to suit every budget. You'll need to budget for lunches and evening meals - when I travel to the states I tend to allocate a similar amount as if I was eating out in the UK and I haven't run out of money yet.
In addition to meals you'll need to consider whether or not you will be joining any of the optional excursions, you may be able to book some of these before you leave the UK (2015 excursions are not yet in the system). Finally you'll then need to just think about any souviners that you want to bring home from your trip.
Hope this helps a little.
Kindest regards,
Julia