it depends
if you want to convert it to a single speed, it depends on the specifics of the bike. If your bike has horizontal dropouts, the conversion can be nearly free (or under $30 at least). All you really have to do is remove all the un-needed parts (Shifters, derailers, and extra chainrings) shorten the chain, and then determine if you have a free-wheel or a cassette. If you have a freewheel, you need to buy yourself a new single speed freehweel. If it's a cassette, then you need to pick a cog you like, and buy some spacers.
If you have vertical dropouts, then you need a chain tensioner, as well as the stuff above.