It will depend on what sort of shifter the bike has, and what needs to be done to it. A broken shift cable would cost under $20 to repair, a worn out SDI Brifter might cost $200 for the parts.

If you don't know a lot about bikes, you will be much better off buying a bike from a bike shop rather than craigslist or a department store. You will get better quality, and someone who can help you determine if it is the right bike for you.