I use a Deere 2210 compact diesel with a 4' bush hog.
Not very big equipment, and only do small areas - up to 2-3 acres at most.
I've found that I have to charge by the hour, mostly because of unseen obstacles, and usually having to double cut most areas to get everything chopped up fine enough. Also, in some instances, the area being brush hogged will become regular lawn area eventually, so that sometimes takes even a third cut, plus cleanup.
I am in a very rural area, and pricing is tough. I try for $60 per hour, but that is sometimes hard when people can hire a backhoe w/operator for that same price around here.
On the other hand, I really don't have any competition in this area, unless someone is close to a farmer who will road a tractor to the area needed to be done. My setup is small (tractor), but at least I am mobile (on the trailer and go).