When I did the trip i breakfasts were included so meals wise it was a sandwich and drink at lunch $10, maybe same again for coffee, cake, ice cream, etc. during day (another $10max).
Evening meal with couple of drinks i allow $30 including tips but it really can depend on were you go from posh restaurant to small bar.
So would recommend $50 per day as a minimum.
The rest depends on you and what you want to do.
You do have some free time for example i remember getting a bus to the south of Chicago to the Museum of Science and industry which was around $15 if i remember, also visited the Civil war museum & mardi gras museum in New Orleans which again cost but cant remember how much.
Also visited a number of music clubs which had a cover charge (plus a couple more drinks)
A lot also depends on what souveniers/presents you buy.
Then not forgetting the coach drivers tip at the end of the holiday.
On another note if you visit a large walmarts as we did (in Arkansas) you can pick up some real clothing bargains so have room in your suitcase (eg, wrangler and levi jeans for $20-$25).
Personally I probably average about $100 per day but know lots of others didn't come to the live music clubs so would spend less.
Suggest you take your minimum estimate and a credit card which can be used anywhere in the US.
Mick