Water heating pipe leaking inside wall.......?

Noticed water leaking through the wall where the hot water out pipe enters into the wall from the water heater. looks like it is leaking from the elbow of the copper pipe right where it goes down into the wall. Now i have to tear out the drywall to replace the elbow and then repair the drywall. Does anybody have a clue how much this would cost to repair? (if I hire a plumber) Thanks