Here is what zygor support told me in a email

[davidm]

There is no difference in the pricing of the bundle to upgrade all guides at
once and the price you pay to upgrade them separately. The bundle is only there
to give people a quick way to upgrade all of their guides with one purchase at
the same price they would pay if they upgraded all of them separately

[Cartman]

Thanks for the info, so I only need to wait for the option to use the Diablo III refund to reduce the upgrade price.