<<That GS has to be aftermarket as Suzuki doesn't do a kit containing piston and seals for calipers>>
That was the retail price right from the Suzuki parts book at Philadelphia Cycle Center. They ordered it from Suzuki, and got it in two days. I also ordered one caliper kit without the piston. I got the master cylinder rebuild kit from an eBay supplier for $27.00.
So far, I have been having very good success with getting OEM parts from Suzuki for the old GS's. I have two customers with 1980 GS850's, and I own a GS450.