The AF-S 28-70/2.8D f/2.8 has been out of production since 2007. It's an older version of the lens that's more or less be replaced by the newer 24-70. Nearly every lens that's been superseded tends to cost less than newer replacement models, especially if found used (when it was brand new in 2002, the 28-70/2.8 cost $1400, which if you cost adjust, comes in around $1850 in today's prices, so it also was not that much cheaper new).
Not only was the zoom range extended at the wide end, but the optical design was overhauled and is not as similar as you might think.
You can find a block diagram for the 28-70/2.8D on mir.com.my (scroll down a ways--it's next to the MTF charts)
And one for the 24-70/2.8G on the Nikon website.
Not only does the new lens have one additional ED (extra-low disperson--corrects for C/A) element, it also has two additional aspherical elements. This undoubtedly added to the production costs as well.