While some people spend money like crazy of buying tons of links on high PageRank websites, others work their way slowly by swapping links, promoting their websites in forums and submitting to free directories.
Those who pay for a PR link normally only do so to increase and help their own ranking in Google, rather than generating actual traffic. Eventually, developing a higher Page Rank will help their site to appear at better positions in Google and Other Search Engines, and drive more potential visitors from SE. But, it will happen only AFTER their own PR will grow.
It’s important to remember that one link on PR7 will not improve your ranking dramatically, you need hundreds, if not thousands of back links to pull your own rank higher, and the question is, will you be able to afford it? Assuming that an average price for a PR6 link would be no less than 5$, and for PR7 no less than 15$ (by the way, good luck finding one for that price! :D), and, as we already said, you need about one thousands of them… It comes to a lot of money, ain’t it? Multiply 15$ by 1k and you will realize that there is nothing wrong with PR3!
Personally, I never hunted the PR, not saying that I’d not want to see my main site – LoreleiWeb.com becoming PR8 one day, but I’d never pay for a link on a particular PR, unless it will guarantee me real traffic. Your best bet would be, if your are ready to invest in adverting, to find a site that has both high PR, massive traffic (above 3k daily) and is related to your website’s theme. In this case, you don’t pay only for improving your ranking, but also pulling visitors. It worked for me, and I hope will work for you.