it's a 10 year old diamandback sorrento 21 speed mountain bike. the right side gear shifter (1-7) doesn't work, and the bike generally needs a tune-up. the local bike shop wants to charge me $75 for an overhaul (whatever that means), + other repair costs, totalling about $110. i don't even think i could get that much for the bike if everything was working fine. advice?