Hi,
So Numista's Rarity Index (NRI) is calculated from how many Numista users have that coin. A 100 means literally no other user has that item, or at least has bothered cataloging it.
https://en.numista.com/help/what-is-numista-rarity-index-nri-79.html
However, that doesn't mean it is actually rare. There are tens of thousands of collectors who aren't on Numista, if not more.
On your specific “coin”, it is filed under fantasy coins. Numista users, in my experience, tend not to bother with such things so it will skew against them & make the NRI very high.
It seems to be noted that only 3,000 of these were issued.
I've actually managed to find a full set of these coins on the bay. Boxed, with all 9 denominations, COA etc, they're asking £55. I can't find a sold set, but other Vatican City set issues are going between £15-£35.
So a single coin would be worth ~£2-~£5 or so. Maybe less, I'd rather save up to get a full set issue rather than an oddment one-off.
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Top & bottom of it is your coin is rare. Only 3,000 in existence and not a single Numista user even owns one. So “supply” is low.
However, it's evidently not a hugely desirable collector set. And as such, demand is “low” which makes it fairly low value.
You'll probably see this scenario frequently if you have modern “collector” coins, or probably any “collector” items. It's artificial scarcity which many serious collectors will simply realise & ignore.
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Anyway, welcome to Numista. Hope you find it helpful & interesting. If you do have any questions, do just ask. One of us will get round to replying (eventually… 😛).