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costs
I've travelled solo about 8 different trips varying from 10 - 16 days & spent anywhere from $500 - $1000 U.S. TOTAL (fuel included). This is probably Extremely Low & you should count on more than that.
I sleep in the car with a sleeping bag at well-lit 24-hour truck stops (free) or at national/state park/KOA campgrounds ($10 - $18 per night usually), stop at grocery stores for fresh fruit/snacks rather than restaurants, perhaps one restaurant per day (breakfasts are often large portions & cheaper than other meals). I don't "shop". I avoid the city life and rather take the scenic routes enjoying the national parks, etc. Showers are usually extra $2.50 - $3.00 at the national parks or campgrounds.
The above prices are probably extremely low for most people. However, each trip does include (for example) a jeep tour at Canyonlands or Capitol Reef, horseback ride at Bryce Canyon, smooth-water rafting at the Grand Canyon, or some such reserved event(s), and a national park pass ($50).
Don't forget to take car of your car. Be sure to be up on understanding basic maintenance (so you can do yourself or not be "duped" at a garage who can tell you are tourists). Be sure to stop for car oil changes as needed on your trip.
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Canada
If including Canada, you'll get $1.50 Canadian money (approx.) for every U.S. dollar.
I agree with Angus about the nights remaining hot in the lower States. We were in the car with windows down in Phoenix AZ last year sweltering in 90 degree heat overnight, debating on whether to go find a motel but we were too "cheap" and just had a restless night instead.
Also - depending when you are going, we've also experienced the opposite in April/May I think --- when our cheap sleeping bags were not warm enough in the Arizona desert & we kept finding more socks/clothes to put on throughout the night.
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Just a word of advice- state parks are generally chaper than national parks. In Texas at least, national forests are 20 bux a night to camp where as SP are 12ish.