Most series of banknotes usually increase in value once the series has been discontinued. It takes time though (the last 2 series won't increase in value as dramatically as the first 2 series, for example). The farther you go back & the tougher the series- the higher the numismatic interest (typically).
Numismatic value usually is dependent on 4 simple factors:
1) condition of the banknote (the higher the more demand)
2) extraordinary banknote features (the more extraordinary or special- the higher the value)
3) how the note was issued &/or recalled* (limited issue or if a series was actively culled/destroyed by banks - the higher the value)
4) demand for the particular series (the higher the demand- the higher the value)
(which is similar to coin's numismatic value)
A lot of collectors think that #4 boils down to how many were issued (the numbers game) but the # issued can be deceptive. Some series have had massive # issued but the banknote was recalled so few survived & the # issued becomes more irrelevant due to its recall (3rd factor). Also, huge # could have been issued & 99% of them were workhorses in a damp environment (which destroyed or nearly destroyed huge #) so this is when UNC EPQ examples can really be hugely in demand even though there are many out there (some common German notes which are very uncommon in UNC).
#4 can be a fickle fish: there were “ca-gillion” of these Zimbabwe 100Trillion hyperinflation notes sold by the bundles (X100) & bricks (X1000). Thousands of collectors bought them. I remember when it all started & they first were sold by Banknote World. Some of the dealers submitted their best examples to graders & started selling the TPG examples for big mark ups. Word got around & demand bubbled so that a $0.50 note suddenly became $5.00 overnight & the phenomenon grew unchecked. Speculators got involved & made sure articles were written about them. I've seen collectors post that they have seen TPG examples of P-91 go for as high as $100.00 (so its become a very popular note)! However, it does have NRI (Numismatic Rarity Index) of 18 which is less than the NRI of Canada's plastic $50 note which has been circulating for 12 years (so there's PENTY out there!) I would be surprised if demand continues to outstrip common sense but who knows?
Here was an earlier thread on what makes a note valuable. Some of the extraordinary factors (such as an “error” or “special serial number”) were discussed/explored in that thread.
Hope that helps!
