Yeah, the time aspect is probably an important one.
Though I think I would make it a bit easier. If your character is keeping a stack of gold handy and accumulating wealth by transforming lead into gold, actually taking the resources skill might be appropriate.
If you don't want to do that, I would make it a "once per session" stunt. That way you can set it up as Sanctaphrax says, but you can just go the "...some time later..." route and don't have to play out the same scene every time. With a limitation like that, there might even be an additional +2 in it, since it's pretty limiting like this.