As someone who owns a mini, I think I can say that unless you are planning on renting a larger machine, you should pass on this project. If you do this for an hourly fee, and the farmer somehow agrees to it, then you that's great for you but lousy for him. And if you somehow agree on a set fee comparative to what a larger machine would charge, then you will be working for pennies and it's lousy for you.
This is a bulk excavation job and there's no way you can compete with a larger machine on a price/volume removal rate. It's just the way it is.