how much is a blown Head gasket 2004 oldsmobile alero V6

Check the estimator on the website for your area. Here south of Ft Worth it's around $900.00 labor for both head gaskets + the head gasket set, usually use Fel-Pro gaskets. He probably won't know if the head or heads are cracked until the heads are removed and inspected. What is the reason for him saying the head gaskets are blown, What is your symptoms?

mech will not know if heads are cracked until they are removed as stated.

well you would have to get a new head and head gasket but call advance and they can tell you a price for headgasket but the head may run pretty penny try using junkyard for the head and take it to be serviced shaved valves checked