Hi Angie,
Always a difficult question as we are all different and its best to break it down. Costs i find given the current exchange rate are very similar to here (although the expected tips in their culture adds a little on).
Meals depends on whether you want to go to a good restaurant or eat in a pub, bit of both i would suggest.
Chicago will be most expensive as its a big upmarket city (think London prices) and things will get cheaper as you head south (bit like heading north in the UK).
Other than meals if you are like me and intend to visit a few music bars you need to allow a for a cover charge to get in ($10ish) and of course your drinks.
Rest is basically what you intend to bring back as souveniers and presents. Be warned in America they don't add the tax on until you get to the till so its always higher than advertised. Rate does vary between states.
Another bit of advice is if you get chance to stop at a large supermarket (walmart) have a look at the clothing prices. Last year could get Levi or Wrangler jeans for $15 so need space in your luggage, got a couple of pair myself and would have got more if had space.
Mick