Look on your bottom bracket shell. There should be a serial number there. Some info on reading serial numbers is here:

http://www.classicrendezvous.com/Fra...10_history.htm

Scroll down a bit and you'll find the info. Here are some old Peugeot catalogs:

http://home.wanadoo.nl/peugeotshow/

Value can range drastically, according to the model. A nice late sixties early seventies PX10 can be worth around 500$ (more if parted, but that's another story altogether). A hi-end Peugeot from the eighties, maybe 200$ on a good day. Most Peugeots are more pedestrian in value, and are worth 25$- 35$, though they can be found for less. I hesitate to attempt to even figure out what model you have without seeing it, as I've consistently been a poor guesser on such things.