Pagerank can have two values. A good pagerank could indicate that Google values your site/content, and it may improve your search visibility. This could make your site worth a lot. How much? It depends on the traffic type and volume.
If you want to know how much your page rank is worth in regards to selling text links, then the higher the rank the more valuable the text link. I've seen text links go for $20 a month on a PR2 and over a $100 a month on PR7. This is all short term value though because once you start selling links it is very easy for Google to detect this, and will then begin lowering your rank...to the point of zero.