Every location will have different labor rates for everything. A few years ago I added a 30 amp box to the side of the garage. I did all the pre-wiring myself and purchased the RV box. I ran all the wire through conduit, after removing paneling inside the garage and up through the rafters and down the other side. I ran about 75 feet of wire, with conduit and everything, cost of materials was around $250.00.
I then called an electrician who came out and installed the 30 amp breaker in the main box and actually hooked up the wires to the breaker box and the RV box outside. His job was easy, just attach the wires and pop in a new breaker. Everything else was done. He charged me $100. I then reinstalled the wall paneling and cleaned everything up myself.
Money well spent since I didn't want to touch the inside of the main breaker box.
A couple years later, I added another 30 amp RV box, near the junction where my electric meter is. I did wire that one in myself. Total length of wire was only about 18 inches!
If you dig the trench yourself between house and garage, or garage and location of the RV box, run the conduit yourself, do all the pre-wiring yourself, and then just hire the electriction to hook up the wires, you can get it done a lot cheaper. If you hire someone to dig up the ground, your talking much, much more. Do as much as you can yourself. Everything you can do yourself will save you more.