When we travel, rates have generally been between $35 and $50 / night. There are less expensive places, but they often are less attractive. I find KOAs to be high-priced for what they offer; but then again, some provide facilities for younger children, so for you that might make more sense than for us.
At busy parks in or near Yellowstone, the Grand Canyon, and Mount Rushmore, you may find prices are higher and that reservations need to be made way in advance. Same thing at many Colorado campgrounds and resorts.
A lot of people stay at Corps of Engineer Parks (COE). Some have full hookups and some don't. Some may take reservations and some are first come, first served. But I think the prices are much lower. State parks often are beautiful and reasonably priced. Colorado has some great ones. Texas has quite a few as well, although because they were mostly built in the 1930s, it can be iffy getting a 40' trailer into many of those sites.