Depending on where you are located and assuming you hire a licensed plumber to do the work, we estimate it will probably be somewhere between $200 - $300 based primarily on the mimimum that a plumber will charge for a job (not necessarily because it will take that long to replace the valves).
usually the cost is more towards 5 -600.
Valves cost $100.00 each plus the labor which could take 1 hour each. $400.00 TO $600.00 is a ball park estimate.