How much is a link to my app worth? 
tbgvi: Agree with the others, buying links isn't worth it. Google will eventually catch on and devalue it, so it's a waste in the end.Focus on sites/bloggers where your potential users are. The most useful links are the one's that actually drive traffic directly to your site that end up converting.I listen to/trust Rand from SEOmoz on SEO issues, here's his thoughts about paid links:
http://www.seomoz.org/blog/our-stance-on-paid-links-link-ads