Its successful in most other parts of the country, especially the south east. Usually its a 10-20/car/day deal with some having annual passes of $150-$250.
With walker, it'd require a substantial increase in the amount of 4x4 trail.. Elbe not as much, since its already pretty good for 4x4s (although they too could use more mileage).
Walker would do ok if it had a ~200k/yr budget. It'd pay for an E&E and Maintenance person + some funds for capitol projects and raw materials. Thats about 10,000 visits a year at $20 or 20k visits at $10. Both aren't out of the realm of possibility, especially if the place was larger.
E&E alone runs about 100-125,000/year.
Maintenance person, maintenance materials, equipment...
I would be guessing much closer to 300,000 or more for walker; then there's capital costs. And that's with the advantage walker has with the fairly closed off system.