How much would it cost for a head gasket to repaired on a 1993 Diamante?

Are you changing it yourself?

The parts will be the least costly.
Labor can run from 7-8 hours flat rate for a single overhead cam, to around 10-12 hours flat rate for a double overhead cam. Hourly rates will vary significantly from area to area so you will need to spend some time shopping around to find a qualified shop that offers some type of warranty.
A responsible shop will as a matter of course need to verify the faulty gasket diagnosis so be ready for that charge as well.
Don't be afraid to ask for technician's credentials before handing over your hard earned money.