I'm not 100% decided yet, as I'm still in the "gauging interest" stage, but I'm planning to make it free for anyone to join, and clients pay a small fee to list a project, mostly to stop some of the crap posts.

I realise that by encouraging clients and freelancers to work directly after the first project is bad for Bamboo itself, but I'm hoping that by staying lean and bootstrapped, the listing and transaction fees should be enough to keep the site going.