PR is a formula that Google created that shows the relative popularity of your site, basically the number and quality of links you have pointing to a page on your site determines your page’s PR. The reason PR and SERPs aren’t directly related is because every link is different. The site it comes from may or may not be related to your’s, might have a penalty against it, the link text may or may not have the keywords present in them that you are going for, etc. You will notice that most sites that are high in the SERPs do have a decent PR score, but the PR score is more of a measure of the work that went into getting links in the first place, and if you have a good linking strategy, you are rewarded with good SERPs. PR is more of a historical measure than anything.
In regards to your question, yes a PR5 site is worth more when you are selling links than a PR1 site would be, because if you have a site related to mine, then I would value that link, and there is some evidence that reciprocal links aren’t worth as much. So, buying a one-way link has value to many webmasters. What you decide to charge is up to you, though there are many link brokering sites out there that will give you an idea of what it might be worth.