Mmm, I'm not so sure tracking metrics is a huge help in trying to fathom why people don't buy. I can only talk from the perspective of our site though.
I can see where they enter the site, I can see where they leave etc. but what goes through their tiny minds in the process is almost impossible to work out - unless they e-mail or call you of course.
Product too dear? Not enough choice? Confused by the pricing? Didn't like the color of the page/site - who the hell knows?