I have two thrift store bikes. They're each about 25 years old and are wonderful rides. One of them has been converted to a fixed gear. Many of the bikes for sale at thrift stores are old but not very used and need little more than cleaning up and repacking bearings to be excellent rides.

You should be able to get a good bike for about $30. You may need to shop around and be patient though to find something you like. If you see something you think you'll like, get it. If you decide later you don't want it, you can always donate it back and take a tax deduction for it's fair market value.

If you have an old bike and need some parts, don't overlook a thrift store as a good source. I was fixing up a bike and found one of the crankarms was slightly bent. It worked, but was annoying to me. So I found a mixtie frame Nishiki cheap ($3.00!) and swapped out the crankarms/chainrings, stem, and bars (better parts). I put new cables and housings on the Nishiki, repacked bearings, and cleaned it up, then donated it to my local charity. I invested less than $10, got much more than that in parts value and provided a working bike that some poor kid is probably enjoying.