intake manifold gasket on 1994 Chevrolet S10 Blazer

why is it so much to change a intake manifold gasket? The garage i go to wants $500 to do it.

Asked by for the 1994 Chevrolet S10 Blazer
The labor guide I just looked at says 3 to 3.5 hours labor depending on air conditioning or not plus price of gasket and antifreeze a few hose clamps parts $75.
but the gasket kit is only $20.00