Hard to put a price on them, like most oddities & collectibles - at the end of the day they're worth as much as someone is willing to pay! The value often comprises of two essential elements ...

1. Rarity
2. Demand

Much like a custom numberplate for a motor vehicle, there can only ever be one, and the more sought after it is - the more it's worth! The scenario here is very similar, except unlike a numberplate there's only one that exists in the entire world. Another example is the 1930 Australian penny, there's only 1,500 or so in existence! They're rare, they're highly sought after by thousands upon thousands of eccentric coin collectors, they typically fetch AUS 25K! The minted ones go for as much as 250K!