we've tracked our cost/day when camping. Now we usually look for state parks, FS parks, county parks, prefer more "primative" or "natural" parks vs. Rv parks. and we usually stay at a given park for at least several days as we site see etc. As for costs, not including the trailer cost, (1) it is way less /day than when we travel and stay in hotels (2) less/day than we spend at home. And the relaxation, site seeing, pleasure is worth every dime, or dollar, or $100. It's one of our pleasures of retirement.
Often campgrounds are $6/night (senior golden eagle pass rate) at FS/BLM campgrounds, state/county parks run about $25-35/night and BLM etc. dry camping is $0.