Costs depend on many factors. Especially how far you go (fuel costs), how long you stay, and where you stay at.
Dry camping, boon docking, or independent camping can save you money.
Independent camping is camping without electricity, water, or toilets. If you are at a camp ground without any of these amenities you are dry camping. If you are out in the open on BLM land or National Forest or your buddies farm, I would call that boon docking. There is a lot of information on this type of camping on the web (especially on boon docking).
BLM land is much more common west of the Mississippi but National Forest can be found in many states. The northeast has a shortage of both unless you go to Maine.
I do not boon dock much because I usually prefer a place with showers. If swimming was available, I would be fine with being without electricity for a few days. I start missing it after two, though. I take a few cheap power banks to recharge my phone and tablet when I am off the grid.
Campers discounts: I use Good Sam’s. I have used Passport America which has a big discount (50%) but there are fewer choices than for Good Sam’s and I don’t find the choices as desirable (some are fine but others seem to be low rent trailer parks).
Summary of costs for each trip