Have you checked pawn shops? They usually have a fairly good selection, sadly enough.

craigslist.com?

no luck on craigslist, but i did find a pawn shop on guadalupe so we're gonna try there.

Also Goodwill.
In the front in the silent auction they usually have pretty sweet bikes.
Maybe more like 40-60 though...
It is HARD to find a cheap bike in this town, but it is not impossible!
Good luck!

There are always many bikes in front of the Salvation Army on 183. Surely there's a bus line that would you get you to that vicinity?

if you don't mind working some, you can get a FREE bike after volunteering 12 hours at the yellow bike shop (behind wheatsville, so maybe within walking distance of you!)


from their page http://www.austinyellowbike.org/projects.htm :
Earn-a-Bike

WE MADE EARNING YOUR OWN BIKE EVEN EASIER! - Earn you very own bike by volunteering only 12 hours to Yellow Bike (it used to be 24 hours). After your volunteer hours are complete you can fix-up or build your own custom bike from our warehouse of donated bikes, frames, and parts completely for free. You also get to store your own bike at the 51st Street Shop for 6 weeks from the date that you start working on your own bike.

This is a great way to learn bike repair and maintenance from volunteer mechanics and build your own free bike in the process. During your time at the shop, experienced shop coordinators will guide you through bike repair projects. No bike repair experience is necessary. After 12 hours of volunteering, working on project bikes, you will be able to confidently start work on your own bike. Simply stop by during any of our open hours and talk to a Coordinator to get started building the bike of your dreams


edit: it looks like they may have moved, so that may not be an option any more... pawn shops it is!

People often have bikes to give away on Freecycle (google for the address; I don't have it offhand).