The process of brand valuation is more an art than a science, or at least that is the conclusion of research done by 24/7 Wall St. The most well-regarded "brand evaluators" rarely come up with similar figures--except for among the very largest brands. Some of their opinions diverge dramatically.
24/7 Wall St. looked at the brand valuations and methodologies of BrandZ, Interbrand, and Brandirectory. The first two list 100 brands each in their public studies. Brandirectory lists 500. Each would defend its valuations which use different "black box" systems for calculating value. That is probably because all three charge hefty fees for their services.
The explanations for their methodologies are even more convoluted.