I have a 2001 cadillac deville I need to get a head gasket and get it installed how MUCH!!

$$$$. DON'T replace just one. My guess -1K.

Ouch! If it was only 1K!
Short of doing the diag, I'm guessing that there's more to it.
Those Cadillacs are prone to head bolts pulling out and then the head gasket fails.
Sorry... That's a bigger job. The damage needs drilled and rethreaded... remachined. BIG job!

Try using www.iatn.net/shopfinder.com to see if someone in your area is capable of properly addressing your specific needs... again, based upon actual diagnosis. That would be your first, next step...

I'm too old to know all the new stuff. Used to pull Flathead fords in 30 minutes with just a 1/2 " drive breaker bar and a 3/4 socket.