Modding is what defines me as a computer enthusiast. I can't afford the latest or greatest hardware, so modding is one way to express that I'm giving it an 'all-out' effort. I've made my own glass window, case-light, bezel, drive-cages, and custom paint. (I'm in the process of cutting up an old aluminum pot for my next mod.) Nothing has been over $5.00 in parts.
Modding doesn't have to be about aftermarket parts. I personally use left-over paint, scrap steel and plastic, old toys, etc. To me, modding is about using artistic talent and craftmanship, not buying stuff.