Expired domains question

Hi all. I know NOTHING about the world of jumping on expired domains (not even sure what a PR3 is, a ranking I assume?)

I need a decent domain for some social media stuff I want to do and I thought that - perhaps - it was time to learn about such things. A couple questions for you gurus:

1) How does one go about searching for expired domains?
2) Does the current PR of the domain matter and, if so, what level PR is worth chasing.
3) What's the cost of an expired domain (I'm sure it varies but a ballpark).

thanks!
EQP

When buying expired domains be prepared to accept penalties that the site may already
have received from panda/penguin/hummingbird before it expired. That includes bad
backlinks as well.

BHW is full of information about domains, just search and read.