Getting PR6 is a big challenge even we have too many links, But if you keep decent unique content in the web site. Then it won't be a such problem.
Any equation of a similar kind would be a wild guess. Another thing is where do you get dozens of pr 6 pages?
20 PR6 Comment links？ I don't think they will contribute to your pr, if those sites are nofollow sites, these links are useless to your pr
if it was that easy every idiot would have a pr6 site you need allot of comments on pr6 sites to get to it yourself
Do it on the well internal linked website/blog may have odds. If just one page backlink, then no sense.
From a long time of constant hard work. In first place, I do have such lists, but I never thought in selling them. A precious list of more than 1.000 dofollow blog commenting pages (not mainpage PR 6 - comment page ???, but comment page PR 3 - 6!) could be worthless in no time if I'd sell it and the newbies or spammers would start to abuse it! On the other hand there is a system (completely white hat) that enables you to create those high PR pages with your link on high PR/ranking sites. Of cause this is not for people who want to push a button and forget about it; and it won't work from one day to another.