wheel - 6:20 pm on May 25, 2010 (gmt 0)
PR of page * $30 / year
That's not a bad formula, except I'd never accept $120 for a link from any of my many PR4 sites.
I guess what's missing is a factor for industry. PR4 backlink from a stamp collecting site? Probably $120 is a bit high. PR4 from a high quality poker site? Probably cost more than that.