My Fuji Roubaix is about 6 years old now. I paid 600 for it and it rides just as good today as the day I bought it...better in fact, because it is now perfectly fitted for me. If I were starting out, and was serious about biking, I would not consider paying less than 400 bucks for a bike. If you are just going to be a casual rider, spend around $300.
Another possibility is going to police bike auctions...every decent sized municipality has them.
If you want to just tool around town, go to a bike store and be ready to answer questions like: What kind of terrain is around your house? Do you intend to ride this off road or just street ride? How much riding do you plan on doing (miles per week, etc.)? How much biking experience do you have?
If the bike shop salesperson does NOT ask you questions like these, RUN out of the shop--they are more interested in making a buck than making sure you get the bike that is right for you. Also, BUY AND WEAR a helmet. Can't stress that enough.