Assuming you are only interested in road bikes, these are some that I would look into if I was in philadelphia.

Not really any info given
$40 Raleigh
$40 Ross
$45 Schwinn
$85 Ross
$100 Schwinn
$130 Schwinn
$140 Raleigh
$150 Schwinn
$225 Trek

Wow, there is a whole page of listings just from today! I need to take a road trip.

Of course, you will want to check them out and ride them before buying. There's a guide to buying a used bike here. With the money you save, you can take the bike to your LBS for a tune up (You may try asking them if you can see what they're doing. Basic maintenance is very simple and well worth learning.) and get anything else you might want/need (pump, rack, fenders, lights, etc).