Unfortunately, there's no set formula for achieving a higher page rank. What we do know is that search engines view links as votes for your site. Thus, the more links you get, the higher authority search engines give you.
Buying links can be a good way to boost traffic and get quality anchored links. I would consider buying links as a part of my overall link-building plan. You can also get links from,
- Forums
- Commenting on other blogs
- Directories (free and paid)
- Article submissions
- Social bookmarking sites
The market for link buying goes up and down like anything else. I would just look at several past link sales and get a general idea about what I wanted to pay for PR3, 4, 5 etc.