I have not purchased exhausts for any MC, but in general a good place to look is on Amazon. It sounds odd but give it a try - its a good way to get a feeling for going prices.
I have gotten tires, tune-up parts and my windshield from BikeBandit.com. Pretty much all of the MC parts websites will have exhaust systems.
It sometimes takes a day or two for people with the specific experience you are looking for to respond on this forum, so looking around on the MC parts websites will give you a quick idea of prices.
Your mechanic sounds like he know what he is doing. When you have an idea of what the exhaust will cost online + shipping, ask the shop how much they can get the same one for. Some shops charge a lot of overhead, and some are pretty reasonable. For example, when I needed a new battery I found them online for about $30. The only ones the local shop sells are $200. But they have pretty competitive prices on tires, windshields, helmets... So you have to check around first.
Whenever I can I give my business to a local shop, because it is nice to have a good mechanic nearby that I can depend on.
Last edited by KCW; 09-28-2017 at 07:59 AM.