Craigslist ''Clone''

I have a client that wants to know, "Are you able to build a site that would function similar to craigslist? If so, how much would you charge to set it up and maintain it?" I don't even know where to start with that but the money is too good to pass up. This company pays well and pays fast. I would obviously need to sub some of this out. Please reply with a workable plan and cost estimate.