$200...you could go way cheaper than that..that is if the rear wheel is rideable or at least in good enough condition that it could be tensioned and trued.
You could probably convert it for about $40. Get the rear wheel trued (if needed). Then reomove all the derailers cables and shifters, remove one chain ring, remove the freewheel, rotafix a cheap track cog on the rear hub (keep the rear brake), shorten the chain and you are good to go.

Or better yet...remove the derailers and shifters, but leave the freewheel on there and just go singlespeed.