You can't really put it into a formula. If the 500 pages are all full of good unique content and targeted to a very specific thing, then they will often outrank even a very high PR page because of their specific relavance.
If you've got the same amount of info on just 50 pages and the PR is higher, they may not rank as well as there isn't a specific thing that they are targeted to hit.
There is a LOT more to the variable than PR. In fact, I'm of the mind that PR is used very little in ranking a page in the SERPS, but rather it's a measure of power that page has to help a page that it links to in the SERPS - among other things like "depth of crawl" factors and such things that don't effect rankings directly. (Others will disagree, but that's my belief).