Land clearing production with a JD 850J?

I'm getting a contractor to look at clearing about 4 acres(300' x 600') of light to medium brush and was trying to get a ballpark on how long it might take. It would take me too long with my 931B and someone else is mostly paying the bill. Some of the brush is thick as in hard to walk through but not a lot of big trees. Mostly poplar and a few up to maybe 8" or so that I think will come out easy. The rest is pretty light stuff just lots of it. I think the guy has a fair amount of experience and has worked for the county on forest fires. He charges $160/hr for the JD 850J and says he can do quite a bit in an hour. Is it best to get an estimate on hours or just how much to do the job? Depending on how much it is to clear, I need the area leveled off and built up to make a pad for a 1/4 mile flat track oval. I can dig clay with my backhoe and we can get trucks and some smaller equipment in to spread it but if the clearing is reasonable, we'll hire some of that out too. The ground is higher so could be scraped down 8" or so to save on the amount of fill to build the rest up. The contractor also has a 200 size hoe for $130/hr. What do you guys with more experience think?

Land clearing production with a JD 850J?