How much does it cost for a valve cover gasket to be replaced?

Depends on the shop and what parts really are the problem. If it is JUST bad gaskets the parts will run around $30.00 The labor is about 1.5 - 2 hours.

Labor rate of $80.00 - final price around $190-200.00

If you DIY you could save $150.00. It's not real hard just takes some time to remove a few pieces to get the covers off, then clean them and install the new gaskets and reverse the process.

Including cleaning it up first, no more than an hour and a half labour plus the cost of the gasket. You don't say which engine. Many had formed in place gaskets, which, in my opinion, should be replaced with "real" gaskets - aprox $20 for a V8???