Oops, I see that you did agree on the price first. I'd probably get a new person. I don't think the price is outrageous, but I wouldn't like them changing it after we talked. Maybe just get it in writing first. But I'm all for paying a decent wage for labor that I choose not to do. Whether it's house cleaning or yard work.
(Yeah, $50/hour is $100,000 per year if you can move mulch for 40 hours a week!! I don't know too many who could do that, or who get that kind of consistent hours in the yard work business.)