A year or so ago it might have made sense to purchase a PR8 link for $500 per month, but certainly not now. Google no longer provides a boost large enough enough to warrant that expense based on PR alone. The days of a PR8 link making your site a PR7 are long gone.
Google now tends to view high pr links as “purchased” and of less value then they did in past. You would be much better off purchasing three or four PR6 links or ten PR5 links.