A billion comments links for 10 bucks? Don’t do it!
Google anything to do with SEO link building and you will be offered so many different desperate attempts to spam the search engine results pages (SERPS). Links within comments areas is an area of SEO that if done in the nature of good blogging (i.e. your anchor text isn’t ‘Viagra’ and you don’t go hunting purely for dofollow links) can be effective. However, black hat software is available for the public domain (hell, it’s not illegal after all!) for around $50-$100. Inappropriate use where lame comments are prefixed/suffixed with an exact match anchor text risks your website reputation hugely. Take for example Blogger. Within a couple of clicks I can report to Google’s Blog division that the person sending the comment is a spammer. On their database they have the Blogger Profile, the content of the comment but most importantly the URL that the comment links to.
If you are presented with an offer to build links ridiculously easy and cheaply – don’t do it. This is a really basic example of how you could get your website in serious trouble!