Well, it all depends...
If it ain't to dirty and messed up and your repairs guy is not someone who charges for nothin'...
Roughly...starting at $50 and up.
If it's real bad...up to a couple of hundred $s.
If there's smoke comin' out of the unit and it's just about to blow up...just throw it away! As Tek says "We live in a disposable world". BUt I hope that's not the case with your unit.
Make sure that your guy uses compressed air to remove all the dirt and dust. SOme of them like to use other stuff, so it takes them a LOT longer to clean everything out.

-ED-