1991 Pontiac Trans Am valve seals

We don't get involved with costs here because there's too many variables. There's a couple of things you have to consider though. First of all, if the owner bought the seals, why didn't he install them? You have to wonder what else is wrong. Second, if he bought them with the idea of taking them to his mechanic, that's like bringing your own food to a restaurant and asking them to cook it. If you don't like how it tastes, too bad. If owner-supplied parts are wrong, or defective, you get to buy new ones again. When you let the shop buy the parts, they assume the responsibility if something goes wrong.

Worn valve seals don't cause a real big problem. What is the reason given that new seals are needed?

Normally the cylinder heads have to be removed so the valves can be removed to do this repair, and that gets rather expensive, but with older domestic engines there are tricks to work around that. Once the spark plug is removed, a hose can be screwed in to connect to compressed air. That will hold the valves closed while the springs are removed and the seals are replaced. That is still pretty time-consuming but the job can probably be done in three or four hours. With Chrysler engines it is much faster because you remove a single rocker arm shaft with all the rocker arms, then you just bolt everything back together and you're done, no adjustments needed. With GM engines, each valve has to be disassembled one-at-a-time, repaired, put back together, one-at-a-time, then each valve has to be individually adjusted. All of that takes about three times longer.