Depends on the context of the site, domain age, it's outbound links, what are those outbound links about etc. I will pay upto $75-$120 a month for a healthy PR8.
Not really, When you consider how powerfull it is. A pR8 anchor contextual bacjklink could get you to rank 1st on most average competitive keywords.
At least $150 a month definitely. If the actual page with the backlink is PR8, then that is an insanely powerful backlink.