Many commercial websites buy and sell links. Link rates vary by industry and Google PageRank. For instance, it might cost $300 per month to buy a link on a "mortgage refinancing" webpage with a Google PageRank of 7 (out of 10). On the other hand, for a less competitive topic area, or on a webpage with a lower Google PageRank, a link might cost only $40 per month.
You can get an approximate idea of how much a text link is worth by using SEOChat's Link Price Calculator: http://seochat.com/seo-tools/link-price. However, don't accept the estimated value as gospel.
My advice on buying text links: don't do it — unless you already have a lot of experience and conversion benchmarks from buying search engine advertising and contextual links. Otherwise, if you're going to spend money on internet advertising, focus on buying search engine traffic from Google, Yahoo, and MSN first, a strategy that is covered in the chapter on Internet Advertising.
