I spent less than $20/day average on my coast to coast US tour in 2010. Mostly I free/stealth camp and cook my own food. But that cost does include some nights in pay campsites, a couple of hotels, a local diner here and there, subway sandwiches at times, an inner tube & chain, and laundry. Personally, I enjoy the concept of touring simply and frugally, so keeping costs down is not a problem.

Staying in pay campsites can easily cost $20-$40/night and buying food at restaurants can cost the same per day. That said you can spend as much as you want by staying in B&Bs and eating in pricier restaurants.