Cheapest hotel chains are probably Good Nite Inn, Travelodge, Motel 6, and Super 8. If you stay in small towns and not large cities,you can get a room for two (sharing a double bed) for anywhere from $40 to $65 per night. Book ahead on these companies' websites or on a discounter site like hotels, priceline, or expedia for some of the best prices. Usually this includes a "continental" breakfast--which means just coffee or tea, and then a small pastry or some cereal.
As far as food, it depends upon what you want to eat. A place like a sandwich shop or a small diner, or any fast food place would allow you to get away for less than $10 per meal. Figuring that you only eat twice a day--that's $20 plus maybe $25 per person for hotels--or $45 per person (not counting your car and gasoline cost). I'd allow maybe $50 to $55 so you can eat a bit more upscale at times--or have some money for drinks and candy bars along the way.
Sounds reasonable--especially since there is a show here in the US on TV called "Traveling on $40 per day"--where the hostess tells you some good places to eat in large cities or resorts that runs $40 in total meals for the day. (and this is even in expensive places like Honolulu or New York or San Francisco).
If you are coming from the UK, the exchange rate should really help you out--since this means you need to spend only about 25 UK pounds per day.
