I would not buy a PR8 link .
That PR could disappear at any time.
I would put that money in to a PPC campaign. You could also get many more PR 6 and PR7 links for one PR 8!
There are more factors to why those pages rank the way they do....for example the number of out going links from a PR6 pages vs a PR8 page.
It would be better to have a link on a pr6 page where you are the only link vs a pr8 page that has dozens of links. The anchor text will also factor into this equation.
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