Wirelessly posted (BlackBerry9630/5.0.0.591 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/105)

The B-up books into ZE which is mighty close to Z. Since the flight is Z0 there is nowhere to upgrade into. The $510 is likely the difference between B and D fare on that segment.



It is entirely possible that more Z/ZE inventory will show up between now and your departure. You can either keep checking back or use a tool to monitor it for you and alert you when the Z shows up.

ETA: The $510 might also be the difference to Y as Y-up inventory is not restricted.