Face it. Sometimes you can exchange links for free, and other times, you just have to buy a link. But is there a minimum toolbar PageRank number that you should go by before buying that link?
Obviously, this isn't a black and white question. Consider the content of the site. Two of the key questions you should ask yourself are "Is it relevant?" and "Can it deliver traffic?"
But beyond that, there's more that can be had if you consider buying that link now. A site that is PR0 can go nowhere but up. If you believe the site has potential, buy the link now and then just wait until the PageRank increases. You'll benefit from getting the early start.
Forum discussion continues at WebmasterWorld.