Don't get hung up on PR when looking for quality links. Though unfortunately as you have probably already discovered 99% of links are priced up mostly based on their PR.
Far more important things to look at:
- quality of the site (does the information/design look trustworthy?)
- age of the site
- number of incoming links (you can use something like opensiteexplorer)
- quality of those incoming links (are they relevant?)
- number of other outbound links on the page (more links means less juice for you)
There's tons of other things, but the above should get you on the right track. The most important thing is to aim for links which look natural and are relevant. Don't get a link on a PR5 site over a PR2 site if the PR5 has nothing to do with your niche.
I realise I haven't answered your question, but that's cause it's pretty much impossible. Could be worth $5 or it could be worth $5000.