I'm guessing the OP is buying used parts, rather than new parts at a dealership.
New parts, yeah, bikes are nuts- along with filters and exhaust, check out OEM brake rotors some time...
Now, used parts are a whole 'nother story, at least with older Japanese bike parts... Of course there are exceptions (the CBX comes to mind), but I'm parting out a V45 Sabre (VF750S), which was a pretty complex and sophisticated machine for the time, lots of one-off parts, and you can hardly give the stuff away... Seriously, with the time and hassle of putting the parts on ebay, I figured out I'd get a better return on investment by taking them to the metal yard.