Depend on which gasket you mean

1. Valve cover gasket: easy job, gasket is about $20~25, takes maximum 30minutes to replace.

2. Cylinder head gasket: Tough job, need remove the cylinder head and a lot of other stuff. $300~400? No way, usually dealer will ask for $1500 and above.

So yours must be the valve cover gasket. $300~400 for that? A definitely ripe off.