Betts told me a while ago he had a plan for an outdoor off road track for nitro cars. Haven't heard much about it since.
From what I've heard, an off road track requires much more maintenance, wetting it down, raking, and such. I wouldn't know, I race on road.
With an on road track once it's paved it's a matter of keeping it clean. Pretty easy. But paving is big $$$.