When you start a mission (at least with the Atreides), if you save it as soon as you start, your spice credits will be stored in byte 24354 (hex). If you've already made something in the game, the proper position will not be exactly that, but it won't change much. So far, I only noticed a change up to byte 24344 (hex). I took the offset values with xe, an hex editor for dos, which I use to edit all my savegames. And as I said, I only tested with the Atreides.