This column was originally published on RealMoney on July 26 at 9:12 a.m. EDT. It's being republished as a bonus for TheStreet.com readers.Amazon ( AMZN) is a $14 billion company. Borders Group ( BGP) is a $1.1 billion one. Barnes & Noble ( BKS) has a $2.2 billion market cap. Now, Borders and Barnes & Noble will have about one-third and one-half as much in sales as Amazon, so that can explain a portion of the disparity. But then when you consider growth rate -- Amazon at 21%, Borders at 12% and B&N at 12% and then overlay multiples to earnings, you get 44 times earnings for Amazon, and only 15 times and 12 times for B&N and Borders. You see where I am going? Unless you believe that Borders and B&N are undervalued by half -- which I don't -- you understand why Amazon's going down so much. A company with decelerating earnings that has to spend more money than Borders and B&N to maintain that level of sales deserves, honestly, to be selling at roughly half of where it is. That's right, at $13. Now, you could argue that Amazon sells other things besides books, but it hasn't been able to make that much headway in those other ventures. And you could argue that an online company has less overhead and lower costs than a bricks and mortar company. Still, the exercise shows you that Amazon down six -- a short-term overreaction -- certainly makes sense long term and still can't be bought.