So what do you have now? Frame, frame and fork, any additional parts? Without parts at your disposal, buying everything new, it will indeed be very expensive and still probably not that great of a bike.
Some of the MAJOR expenses, if you want to use decent parts:
- Fork
- Crankset
- Shifters
- Wheelset
- Brakes, if they're disc.
Then add in all the little stuff like headset, spacers, stem, bars, grips, seatpost, seatpost collar, saddle, cables/housing, FD, RD, chain, cassette, tires, tubes, pedals, and you've got a small fortune stuck into an old FSR.
You can get the same rewarding learning experience buying a complete used bike and tearing it down and rebuilding it. Then at least you have all the parts. Anytime I get a bike I'm going to use for myself now it gets torn down to the bare frame, cleaned, and rebuilt properly. Part of it is OCD and the other part is that virtually every bike I come across needs this service done.