This is a touchy subject. The white hatters are going to object to buying links and the black hatters are going to agree with buying links. It all depends on the source of the links, what is the outbound ratio, is the content manually written or spun?
There's a few factors to consider before buying the link. My best advice is to ask the website owner if he or she would accept a guest post on their site. If it's a private blog network link you're buying, you need to be cautious as they tend to churn and burn. Rank fast, burn out. It's better to rank slower but remain stable.