I think this is a worthwhile effort. Learning how to do intermediate bike repairs is not very difficult (I did it) and you can acquire tools as you need them and likely be able to buy them with the money you earn. I would recommend starting by fixing flats (your cost for a bulk patch is around $0.15, the bike shop charges $10+) and adjusting brakes and gear shifters (about $35 in tools).
You can read about one way to do it in much greater detail here. I have used my repair skills primarily on my own bikes and volunteering on a needed basis for a shelter.