As they say it's unfair to tar all with the same brush, but years ago we found KOAs to be extortionately priced in the areas we were booking into and very much nickel and diming with add ons - accordingly we have avoided them for more than 15 years since our first year on the road with them we felt wasn't fair bang for our buck. For us it was a couple of times bitten more than twice shy.
However, $40 for an overflow whilst seems extortionately high is hard to judge based on not knowing the location, was there a special even in town or generally on supply and demand. Also was is the full story did they double book your site?
For example you can stay on a tarmac carpark in Chicago for $22 a night plus $8 reservation fee, so $30/night, which would be very similar to an overflow situation.