The price I'd be willing to pay would be based on

- how the site gets it's PR (.edu, .ac etc are preferred over .ffa or amazon aff links)
- how many inward links (one link from a PR9 is less important to me than lots of lower PR links)
- minimum owner supplied PR (I'd prefer if it was getting PR from third party sites rather than sites you own)
- the subject area it's in (hard disk recovery, webhosting, creditcards, being more valuable to me than anime or free image downloads)
- the amount of content on the site in terms of articles, reviews, advice, tips. (Sites that look spammy/have scraped content etc. are definitely out).
- the length of time the site has been up

I appreciate the issues with PR; it's lower relevance is not material for my current purposes. If a page on the site has a (toolbar) PR of 7, I'm interested. I could go up to about $25,000. Payment will be by escrow (of course). Full confidentiality is assured. I'd be happy to sign an NDA.