depends on what amenities you need. If you can "dry camp" without hookups or just electric, then public lands or boondocking are definitely free or cheaper. We prefer commercial campgrounds for full hookups. We don't care about pools, etc. I try to find campgrounds for $25 per night with full hookups. Weekly rates are sometimes cheaper. Use discount cards like Passport America for midweek bargains. If you are eligible, get the America the Beautiful national park pass for seniors or disabled for half off national park campgrounds and corps of engineers parks.
