I just finished doing the bookkeeping on our trip. I thought some folks would be interested.
In 77 days we logged a little over 12,000 miles hitting destinations in Washington, Oregon, California, Montana, Nevada, Utah, Colorado, Arizona and New Mexico. We averaged a fraction shy of 20 mpg in our 2001 Tundra and spent $2100 plus a bit on gas. That's an average of pennies over $27/day.
Mostly camping and prepping our own meals, we spent an average of $24/day on life support, including a few meals out, a few nights in cheap hotels, and a few nights in formal campgrounds.
Those figures are all rounded to the nearest $, but our trip cost work out to $51 a day for transportation, beds and meals for two people.
No our digs weren't fancy, and sometimes they weren't all that comfortable. But we ate very well (try stuffing fresh aneheim chilis with cheese, wrapping them in bacon and grilling them over the fire, eg), and we had good wine most nights. Gotta love the cheap case prices in California, but I'm sure the three cases in our truck cut our mileage. Some tradeoffs I'm willing to make.
And that's the bottom line for most folks I know who camp. If you do your own cooking too, you can usually cover all your gas, bed and food costs (including good wine) for what you spend for a very moderate hotel alone without the camping gear, and still have to buy gas and eat. If you're driving you're own rig rather than renting, costs drop even more. We could cut the costs even further by driving something cheaper, but at the cost of comforts in camp including cases of wine.
For most people I know, the tradeoffs boil down more to comfort choices than money savings. Hotels and restaurants are more expensive than camping, but free you of a lot of housekeeping. Large camping setups are more expensive to support than small ones, but offer a whole lot more comfort in exchange.
I'd have cut costs a bunch if I was driving a 40+ mpg sedan and camping light, but a lot of those cost savings would have occurred because my wife would have stayed home. That's a compromise I'm unwilling to make.