Between $1000 to $1500. It's a big job because most of the dash has to come out. It takes about 5-6 hrs to do.
replace the 2 o-rings they cost about $3 each
a new heater core will cost about $250 - $300 depending on where you source it from.
If you buy the part yourself and do it, it will be a fraction of what labor will cost on the vehicle. I suggest checking the net on how to replace it, then find it at a parts house.