This scenario is flawed because there is nothing there about factors that affect the amount of PR that may flow, if any. GoodROI mentions two important factors, the placement of the link and the fact that sites selling links may be penalized and not pass PR at all. But those are just two of many more factors impacting this calculation which in my opinion will not result in an accurate result. Furthermore, even if you could pin down a number that number would not be useful for determining the worth of the link because there are other factors that determine the worth of a link.
Re "outbound internal" links on a page, as well as general outbound links, we know that not all links will pass the same amount of pagerank. What we don't know is exactly the amounts of PageRank that flows out. For instnace, 20 links does not necessarily mean that each link flows out 1/20th of the PR of the page. Some links may be flowing less. Indeed, the amount of PR flowing is likely dampened, across the board so it could be even less.
Then you must take into consideration that the PR displayed on the toolbar may not represent the real PR. But to really screw up all hope of determining how much PR is circulating you have to consider the relevance of the verticals of the backlinks of the sites linking to the site you are considering.
This is all to say that basing a link purchase decision on the amount of PR that may circulate from a site is not possible and in my opinion is the not the correct way to judge the worth of a link. There are many more factors.
GoodROI mentioned traffic. Traffic is a symptom of a healthy site that is doing something right. If you must use a metric one metric that is more important than dividing the amount of links on the page is to visit a site like QuantCast.com or Google Trends for Websites and see what their traffic estimates are. In my opinion, the true worth of a site is measured in traffic. Good traffic is a symptom of a site that is favored by Google.