Hey hey
you know css zen garden (
http://www.csszengarden.com/) has something going this reminds me of.
If it's truly to be a test of design's ability to seduce/convert traffic, then there should be no variables but the design & tour.
So price/range stays the same - in fact the way css zen garden does it they have something like 80 different designs (all called from .css files) and one html page that is affected by the different designs.
I'm playing with this myself at the moment... pretty powerful stuff css, though it requires that one's brain rewire itself away from html ...
did you go see the site?