Driven many a mile of t-post by hand and never charged anything other than by the hour.
Are you building the entire fence or just driving the posts??? If you're building the fence figure it into your overall bid/price. If not, sounds like you're hiring on to the help the builder.......at which point you pretty much need to work with what he's offering you.
You're not getting "taken" by working by the hour on this. Soft ground........work is easy. Hard ground......you'll make more.