I admit that 12 hours seems on the high side, however....
Years ago I admitted to my A&P that I should never complain on some of his "high" quotes. Why? Because I started doing most of the work myself and it took longer. Even after number of years working on my plane.
And while lower cowl removal takes time, installation is really PITA. It's the best to have a helper (more billable hours) and attaching RAM air cable is very difficult due to 2nd landing light my plane has it there. You might have an oil cooler on that location which might be worse, IDK. Then, there's that infamous $700 Intake duct attached by 6 bolts to the cowl that are difficult to reach. I can easily spend up to an hour to disconnect or put it back on. Years ago I made a deal with my IA not to remove the lower cowl at the annuals to reduce the pain. But still, I have to do it every now and then and it's NOT fast. Not on my F.
I recently installed the new Skytec starter after the old one failed. It was 149-12LS, same as the old one and I needed to (partially) remove the baffles, which means removal of the tension rods, drilling old rivets and number of fasteners. I didn't have to modify the baffles, though. Access to couple of bolts was very difficult, other two were not that bad but it took time. I've done it in couple of days (week apart while wafting for starter) and I'd like to think it was less then 4 hours for removal. Installation? Did I say it's PITA? I took me almost full day on the weekend due to access and trying to fit it without removing front baffles. My mechanic had it inspected and did the paperwork.
My advice? See if you can help and do owner assist annuals on your plane. Over years your IA should let you do more work, you can learn more and save some money. But mostly, appreciate the time and pain needed to put some things in order. You can start by removing lower cowl, if needed for exhaust inspection or such. Luckily, we don't need to take it of for simple oil changes.
Good luck.
Edit: Nocking off 4h of your bill seems like a good deal. I would stick to this mechanic/shop.
Cheers