oh, and don't forget the supply/demand factor. if you have several thousand notes and a million people want it, you'd have a pretty valuable note. if you have several million notes, and only several thousand people want it...they'd be dirt cheap!
ayatollah notes are interesting, but I doubt they'll be skyrocketing in value anytime soon. they're sorta like the saddam hussein notes. even if there's a big interest in them, there's more than enough of them available, and people are selling them left and right.
if you browse e-bay and you see a certain banknote appear more than 10 times, it's safe to assume that there's plenty of them & you won't get rich investing in them. the same thing applies to any banknote people are selling in bulk. dealer lots of 10, 20, or a hundred pieces of a certain note = not a rare note. this goes for collector lots too. the kind that advertise 20 different, 50 different, 100+ notes. almost every single "collector lot" I've ever seen for sale is full of the "more than 10 times" notes.
for collectors, all of this is no biggie. it doesn't matter to them. but if your goal is primarily in investing, you should steer clear.