Anything that can be given freely can be bought. Links, user behaviour, whatever. Look at the 'worlds oldest profession'. To avoid spamming, a SE needs rely on either a *huge sample, (hard to buy enough votes to matter) or *very small sample (votes you can trust) JMO
Currently we set up a project called "openpv" (open page value). The first step is the design of an algorithm which calculate the value of a single web page (something like the PR but including category factors and more). We would love to see more people telling us ideas and opiniions...