The exhaust manifold gaskets themselves are not that expensive however the cost to replace the exhaust manifold gasket by a shop is what costs the most.
The higher cost in replacing the exhaust manifold gaskets will be in the labor charges by the shop and the time it takes to remove the exhaust manifold gaskets and replace them.
On some vehicles the exhaust manifold gaskets are easy to remove and replace while on other vehicles the exhaust manifold gaskets are more hard and labor intensive to remove and replace.
On my truck for instance the exhaust manifold gaskets are easy to get to and replace while on my car they would be harder to get to and replace.
Most shops though can replace exhaust manifold gaskets within a day depending on how hard they are to get at.
The exhaust manifold gaskets costs from $10.00 to $40.00 each or $20.00 to 50.00 per set of exhaust manifold gaskets but the shop may mark the price up of the parts.
Then the labor cost would be around $100.00 to $300.00 depending on the shop and the time it takes to replace the exhaust manifold gaskets.
I had my cars exhaust manifold gaskets replaced by a shop because I didn't have time to work on it and the shop charged me $250.00 including parts and labor plus taxes which was well worth it because the exhaust manifold gaskets on my car were a pain to get too and would take me days but the shop had it done in 1 day with the proper tools.