It looks like it's $30 per person per day.
I scared myself pretty good at the other park down in Southern Maryland so I'd be loathe to ride something described as an ATV trail on my DR.
The other OHV area, also fee based, is called Taskers Gap and has quite a few miles of trails with quite a bit of rock garden but some of it is quite fun.
Per request, the track of our offroad trip earlier this year.