I think it also depends on how much traffic you can drive to the linked site. Also, you don't want to sell the links based on PR, you can get in trouble with Google for that. right?
Paid links are not worthy enough.It is better to get backlinks from high PR website in organic way. It may be time taking process but get better result and high traffic to your site.
Buying links on a high PR site is not a good idea. Try to make natural link from your self.
Buying links are forever worst thing you can do for your website. Google always say to always be natural and this one is definitely one of the most un-natural thing.