The complete chaos in prices of links with definite SEO parameters (PR, OBL, age) usually results in unjustified waste of SEO budjet. The blogpost on crazy jumping SEO prices is devoted to this problem and its possible solution. Your feedback will be highly appreciated.
Two simple solutions: 1. Concentrate on SERPs rather than PR. A huge majority of sites offering links for sale have no decent SERPs ranking. If Google or Yahoo/Bing don't rank them in the top 10 or top 20 for their main keywords what link juice is there for me? Any site can have a decent PR, but not every site can have decent SERPs. 2. Don't buy from large scale link sellers. If someone offers dozens of different sites in different niches it just shows that they are there for the short term to make a quick buck. They really don't care much about their serps and the quality of their sites.