First I would get bonded so you can show your covered personally in case of accidents and also that the landowners livestock and property are covered. That shows you care about them too.
Next before getting into the particulars of what you want to detect... offer to do some work that they might need done. Fixing fence, gates, any repair of outbuildings, mowing, brushhog work etc. Anything at all to show you are willing to be a friend first.
I agree about sharing or giving a few finds to the owner. You also could have references from other land owners who let you hunt showing you are responsible and honest.
After you build up some trust and a friendship then broach the subject and details if a larger amount is recovered. Get everything in writing.
Being Bonded Goes A Long Way in this day and age. Check with your insurance company, they are very reasonable.
Good Luck!
Kace