As for labor costs, how different are the seats you have from what the 300 had in it? Aren't the basic seats the same, with buckets starting in '57?
Is that price including a complete rebuild of the seat structure/frame with new burlap, foam, etc.? Include door panels and such, too? Or just the seats?
As with paint jobs on older "collector" cars, just specify that you want an "O.E.M.-spec new car job", NOT mentioning anything about "show car", "custom car", or "collector car" in the conversation. What that means is no puckering on the seams, the sewn pleats are all straight and index with the other parts of the seat assembly, just like it is on a new car of recent vintage. IF they do warranty interior work for the local new car dealers, they'll be able to do those things as a matter of course, I suspect. That gets the upholstery itself taken care of.
The condition of the "underlayment" of the upholstery will probably need some attention/upgrades too, I suspect, but that can be a variable situation. New zig-zag springs, as needed. New burlap to cover them. New foam on top of the springs, etc. No real rocket science there, just the cost of the materials and labor to fix them, as needed.
From there, that leaves the cost of the upholstery fabric itself. Be sure to get the "O.E.M. spec" stuff there, too, for best results. You can probably shop the different vinyls to find a grain that close-matches the leather in the "real" 300s, too! Plus matching the correct three colors themselves, as close as possible.
I'll concur that if somebody gives you an estimate that seems too high, that's a clear indication they don't want the job. Look elsewhere. Be sure to get the job priced by the different aspects of it. The covers, the seat frame/underlayment, and installation. That way, you'll know what each segment costs, to make a possibly better decision of which shop you decide to do the work.
Enjoy!
CBODY67