KOA's are like Holiday Inns used to be, or Howard Johnson's. We pretty consistently use them when on the road because of the usually dependable minimal standards. If there is a high rated alternative in the area, we of course will take that. But many times, it is "Joe's Slum" vs KOA.
In the end, my benchmark is pretty much always - what would we have paid if we didn't have an RV and this was a Motel 6? $40-$65 in a tourist area is still a deal, especially if it is for a shorter time. For destination stays, I'll look harder for alternatives, but either way, campground pricing is generally the least of my concerns.