2001 3.5 Aurora - Rear Valve Cover Leak

I just purchased this 2001 Aurora with 27K miles and now the rear valve cover gasket appears to be leaking. A dealership that I spoke with said that the head gasket is known for problems and leaking on this engine also. How much should I expect to pay for a new rear valve cover gasket replacement? What is the best way to tell if the head gasket is leaking oil also?