I was thinking along the lines of pages-per-visitor > X, time spent on your site etc, not necessarily measure conversion by completing the sale online.
Also it'd be comparative, ie if clicks from different Adsense sites to the SAME advertiser result in very different behaviour, the one providing "better" leads should be rewarded more.
I think letting the market's supply/demand take care of the rest is best.
And allowing the Adsense publisher specify a minimum EPC, e.g. no clicks under $0.10 in this "channel" etc
Btw this will make things a bit more "fair" in terms of the "branding" benefit of publishers due to all this exposure, which isn't going to be paid, as the system for publishers is ALREADY pay-for-performance, at least in THEORY.