The gasket alone is actually a very inexpensive part (approximately $10-30), however the job itself is labor intensive - expect to spend $1-2k, depending upon whether the gasket replacement is...
Actually the part is inexpensive, but the labor is going to cost you from $700 to $900, also depending on the type of car you have. If you have a foreign car, it'll cost you more.
I found this out by accident when trying to order a header (exhaust manifold) gasket when he quoted me a head gasket by mistake... Per a Mitsu dealer, they'll charge 80 bucks for a head gasket. I...
The gasket itself is about $100 bucks, but all the work that goes into removing parts to get to the head gasket to remove & replace it, and then put it back together is what runs you into the...