Your discounts and mail in rebates do not count because of extra labor, time/value of money and I suspect there is a limit on the number of rebates per customer thus not 'in volume'.

I am trying to find your exact machine for comparison, the best I have found is: http://pcpartpicker.com/p/1W43u for $328.92 with promo discounts of $50.   Promo Discounts do not count, so $378.92.

When I replace the drive with a USB stick I end up with about $350 then I add labor I get to $400.   Then I factor in 3.1 vs 3.9 Ghz and that is not a viable option.  Even with the promo codes this machine does not compete.

Unless I missed your link to the $312.94 machine.