A $39 user interface test is not going to give you the same results as you would get from hiring a conversion optimization / online marketing pro, period.
A/B testing (@seoswat mentioned) is one way of testing graphics, landing pages, text, headlines, colors, etc. Once you've built a solid foundation, A/B testing is a great way to refine it.
However, in many cases a website is needing serious overhaul to the design and/or information architecture that no amount of A/B testing will reveal the issue.
The process used at a lot of the larger digital agencies is to develop typical use cases of how buyers use your site [what do they search for, how to they approach it, etc]. Out of those use cases will come buyer personas, taxonomies and clear picture of what people are trying to accomplish when visiting your site.
By examining the current site state of affairs, in light of the above, you can find stumbling blocks to being able to accomplish those goals. Then, you can fix those issues.
The $39 user test is to 'real' conversion optimization what Scalectrix are to Formula 1, an inexpensive replacement that may give moments of pleasure, but nowhere near the same results.