Insurance has been covered.
Many firemen and school teachers do very well in the summer brush hogging and mowing, in the winter snow removing, and in the spring tilling. The safest rule for self employment with a tractor is to stay above ground (except of course the six inch or so dig tilling). When you start digging you start looking at below ground hazards and overhead problems.
The how much to charge problem is pretty much removed by bidding a job, as nobody is surprised when it comes to pay up time. The dollar decision is yours as to whether a fast nickel or slow dime. BUT, don't forget to put back the routine maintenance and occasional breakdown costs.