blown head gasket on 2000 mercury sable

I have a 2000 sable with 94,000 miles on it and i am the second owner.Cylinders 2 and 4 had been missing so i put new plugs and wires on it but cylinder 2 was still missing.So i took it to my local Ford Dealership .They said it was a blown head gasket or a cracked head.Is there any warranty on these parts should they blow at 90,000 miles?they want 700.00 bucks just to replace the gasket it is 1200.00 if it is the head......