Across the street from my house is the largest inner city park in the country (yes larger than Central Park) complete with mountain bike/horse trails, a creek, jogging paths, etc.
In order to use the bike and horse trails, you need a permit. $20 a year. You don't have to pay to use the multi-use trail down by the creek no matter what "vehicle" you use.
The money is uded to maintain the trails, and I'm very supportive of it. Mountain bikes and horses erode the hell out of the trails, and ours are very well maintained despite the thousands of users daily.
Also, while not directly related, alot of ski resorts, such as Whistler in BC, are creating mountain bike trails on the mountains and selling them just like a ski resort. You pay for the day, buy a lift pass, and ride down. Fun, fun, fun.