As has already been said, it will depend quite a bit on what section of the country and what you require from the RV park.
We are spending the late summer and fall in a great little place on the Florida Panhandle. It is on a lake and has only about 25 sites total. Quiet, great fishing (a must for us), and $225 a month plus electricity. Of course, it is about 10 miles from the nearest town (across the line in Alabama) and quite a bit further from the nearest large town or city.
We've been looking hard for a place further south in Florida to spend mid-winter but the prices are nuts.