There's a big sign on the way out as you are leaving, can't miss it. Well, I suppose you could miss it, but only if you were totally burnt out from partying all week in the desert.
Oh wait...

Hi
Contact me via email if your looking for a cheap bike pre-event. I can also help with donations. If you are unfamiliar with me, I am the bike manager for the event. All bike ?'s are welcome at travis@burningman.com

There is also the Yellow Bike Program:

Burning Man also runs a community Yellow Bike program, where a fleet of bikes are made available for participants to borrow for one-time, one-way, no-guarantees use. These bikes are painted green and (ironically) say "Yellow Bike" on them. A Yellow Bike can be taken at any time by anybody, and used to travel to their destination. Once it's been dismounted, anybody else can take it and use it as they need to, and leave it when they arrive … rinse and repeat. Yellow Bikes are a community resource, and are NOT to be repainted, augmented, damaged, locked up, or kept (say, in your camp) for private use. It requires community cooperation to keep this resource alive for all to enjoy!

burningman.com/on_the_pla..._playa.html