Outsourcing link building can be a dangerous strategy, because you will have no quality control over the work done – there's no tangible product to check, and it would be very different to quality check retrospectively. That means you're taking a huge gamble with the integrity and competence of the contractor. Now, I'm not saying that all professional SEOers are liars, fraudsters and charlatans, but it's fair to say that a significant proportion are.
As 4daLUV says, the latest Penguin update has really tightened things up, and it's going to be increasingly difficult for people – particularly those working outside your industry – to build relevant links that carry useful quantities of googlejuice. And you really don't want to waste your money paying people to build links that aren't going to do you any good at all.