To buy links from other sites is also a good idea to improve your site's link popularity. Your don't need to link back to their sites. Most often, it costs $5 per link from page with PageRank of 4.

The other advantage of link trade is since you never need to link back and links are not reciprocal, search engine will put more weight on such links. In fact, as a forever fight between search engine and seo, search engines are aware of the actions taken by seos to increase their sites' ranking. As a result, one-way link is much more helpful to push your site's ranking.

Advice on buying links
1. Links should be text-based. Otherwise, it helps little.
2. Links never be from web pages that is or was penalized by any of major search engines