It depends on how difficult it is to get to the valve... the value costs about $75 to $100, the rest is labor. If the value is readily accessible through the back of the wall, then this is too high.. but if the plumber has to remove and replace the tile or otherwise open the tub side itself, then is not unreasonable.
If the valve is accessible from the other side of the wall, consider opening the drywall yourself and when the plumber is done replace the drywall and repaint yourself.. This should save you some money, if you are comfortable with doing this.
also, this is not a difficult repair to do yourself if you are reasonably handy...
turn the water off,
open the wall from the back side,
remove the value...
Buy a value with the same plumbing configuration..
(Take the old valve to the store and check the configuration of the new value against the old.)
Re-Install the valve
Repair the hole in the wall.