That depends on where you live and how level your property is. My mom has level property and where she lives you can hire someone for $15-$20/hr. So $30-$40 would finish the job. Where I live you are looking at $35-$40/hr. and my property has a good slope to it. There would be a lot more digging, plus there tends to be a lot of roots as well. It would take a good 8 hour day, so $280-$320.
Too many factors to say. They will base it on the size of space, and what type of bricks, as well as if you want a design or not. Ask several people for their prices and compare them! Plus, ask to see photos of their work to make sure it is decent.
You can pay an individual between $20 and $40 for him or her to install a brick tree ring. The amount will be determined by your current location, the type of bricks and the during taken to install the brick tree ring.