I can give a little perspective. I wound up buying a 1 MByte SMI card last November for $170, but you may eventually do better. Shipping between US and Europe usually ruins the deal, so I had mine shipped to my brother in the US. The previous August, I got a 128K HP one for £49 ($75) shipped locally in Britain. Bear in mind that these prices are a lot less than when originally released. They were always more than the calculator (about the same for 128K).
It's a question of waiting, I think. Stay away from the 'built-in battery' types, as these will require surgery when the old batteries stop taking a charge. Also stay away from seller 'coburlin', although his prices are at least three times too high anyway. He also sells 'GRID' cards as 'for 48GX'. These have a nasty low-voltage detector that can ruin the calculator.
There are a number of sellers who have bought up a lot and are dribbling them out at 'collector' prices, but occasionally, someone shows up who just wants to auction one they own. You may do best buying a whole combination of calc and cards from one of these real owners, then selling on what you don't need.
I haven't seen the current manufacturer -- there is an 'OEM' from Singapore, but that again is expensive, and with a built-in battery.
Use lots of combinations of hp48gx, 48gx, hp 48 gx, hp48, etc to search. You do want one for the GX as opposed to 'bank-switched'.
Edited: 20 July 2010, 4:59 a.m.
