About $42 shipped from Compaq for the Ipaq. None of the other handhelds of that generation (HP 54x, Casio E1xx, Casio E5xx, etc.) can upgrade, because they don't have flash memory. Even the compaq solution is half-assed, because it only has 16-meg of flash memory and a 'true' upgrade would require 32-meg, so some things like the media player, etc. will use regular RAM, and I think the Microsoft Word spell checker is not included at all (don't quote me on that one).
So, it's a decent deal if you have an iPaq (I do, and have ordered the upgrade). Otherwise, you're SOL. The *GOOD* news is that this shouldn't happen again - all future Pocket-PC's are REQUIRED to have flash ram to allow for future upgrading.