I tried this and it seemed to work pretty well. I looked over the area that I had to cut. Then I did about 100 sq feet and stopped. (I was using a Echo SRM 23 with a weed blade). For this I ended up charging about 75/hour
Looked how long it took and then figured out from that small piece how to charge for the rest. Worked great the 2 times that I did it. After a while, I'll probably be able to just walk it and bid.
If you are using a brush hog, I usually charge them 10% of my machine per hour. If I use my JD 950, I charge 150 per hour depending on the lay of the land. If it's wet, extra rockey, etc., you can adjust the charge (higher)