Each Google PageRank, which you see as a green bar in your Google Toolbar, has its own hidden weight, which is not shown in the toolbar. In the table below you can see that each PR has 5 times more weight, than the previous PR.
Google PR of the Backlink Page PR Weight of the Backlink Page
It means that if you buy a backlink on Page A following to your Page B, then your page will get the following PR weight: PR weight of Page A / number of outbound links on Page A - 15% = weight, passed to your Page B - this is the formula! (Note: we didn't invent this formula. You can find the same, but more difficult explanation of Google PR formula on Wikipedia and other web resources.)
For example, if you buy a backlink on PR6 page with 25 outbound links, then your page will get the following PR weight: 15,625 (weight of PR6 page) / 26 outbound links (25 existing links, plus your link will become 26th outbound link) - 15% = 510.82 passed PR weight. This weight will be added to your current page's weight. If your page is PR0 (1 weight), then it will become PR3 in the next Google PR update. Why PR3? Link from PR6 page passed 510.82 weight to your page, and if you take a look in the above table, you'll see that to get PR4, you need to have 625 weight. Since your page has only 511.82 weight, it will get PR3.
Now, let's take a look at another example for you to see how important the number of outbound links is. For example, if you buy a backlink on PR6 page with only 1 outbound link, then your page will get the following PR weight: 15,625 (weight of PR6 page) / 2 (your link will become 2nd outbound link) - 15% = 6,640.62 weight. If your page is PR0, such backlink will increase your page PR to PR5 in the next Google PR update!
All this means that when you purchase backlinks, you need to consider not only Google PR of the page, but also the number of outbound links on that page. This is even more important!