It really depends on the niche - e.g. a PR2 link for insurance is gonna be worth more than a PR2 link on obese midgets.
It really comes down to what people are willing to pay, and also the PR of the page itself, not just the PR of the homepage of your site, and if it's a site-wide link or not, along with the number of pages indexed in SE's.
It's debatable but lots of people also seem think forum links aren't as valuable (but then again no-one truly knows Google's / Other SE's algorithm).
May I suggest you try ripping off some poor bastard on DP or Warrior Forum?!