I think the biggest problem for S12 values at the moment is S13 values! You can pick up an S13 for under a grand and there just don't seem to be many people out there willing to pay the same or more for an S12 when they can get an S13 instead
The S12 may have more character and be rarer, but how many Nissan collectors are really out there? Even if there are a couple of dozen (which seems the likely maximum) that's not enough to turn the car into a 'collectors item' in my opinion.
The S12 is a great car
but I reckon they're a good few years away yet from being worth serious cash.