How can you know that the link building that you paid for worked? I guess the end result should be the improvement in the search ranking. Does it really work? I think it can work but only for the SEO tech that knows how to fool the Google algorithm. You have to understand how Google decides ranking to be able to get the links to matter. The best way to find a good SEO tech is by referral. They can make all kinds of claims online but the reality may be much different. Dana Sibilsky is a good SEO tech and I would recommend her.