As a comparison, at least in terms of a stopover location, here's a campground that is about 5-6 miles off the interestate
http://www.benchmarkrv.net/index.html
I've stayed there once on my way to Texas, and plan on staying there again on my Christmas Texas trip.
It's $30 per night, but with Good Sams, I think it was $25 or $28
Concrete pads... all pull through's, Full Hook Up including cable and wifi.
Now, if you have a 50 amp rig, there is a slight up charge, EVEN if you say you will use the 30 amp plug. It was only a couple bucks, no big deal really.
It's a decent place to stop over night. Fairly close to the freeway. So, I would say comparable to what OP might be thinking. Just an example of what you might want to be prepared to offer/plan for. Didn't take up a lot of room. Two rows if I remember correctly.
Mike