There is a HUGE difference between mounting a binding on a pre-installed rail, like the Rossi Z11 and mounting on a "flat" ski and having to drill. There is FAR more work in actually drilling and far more ways to screw things up, hence the higher prices. To me, a fair price to mount on rails would be in teh $20-$30 range and to drill in the $50 range.
I don't for a minute think it is reasonable for them to charge you more because you didn't buy it there. If it were me, and I were running the shop, I'd say "hey, this is my chance to show this guy how good we are, how reasonable we are and to convince him that we are the safer choice." I'd WANT you to do business with me, not drive you away. I don't understand how short-sighted these shop owers are. They should want your business.
For the guy above, Virtus_probi, who gave the plug to Ski Fanatics in Campton, NH, I agree. Those guys are the best. Don and Debbie are longtime Campton/Waterville Valley people and really know how to take care of their customers. Go see them if you can. They're good.