I generally buy used bikes from an LBS for around $200. I have not been able to find stuff (that I wanted) much cheaper. However, for that price I get recognized brands like Trek, Norco or Raleigh, usually 10-15 years old, and if I commute one season on them they pay for themselves. I've actually given a couple of rigid mountain bikes to charity after a year or two of use, but my 80's Panasonic road bike I commuted on from 1993 to 2005 and it's hardly the worse for wear.