Stupid Question Alert

I think numista reflects rarity based on their members catalogues not so much the rarity of any particular coin that you search there
it may give you a sense of whether or not a certain coin us common or not but I wouldn't say it should be your only reliable source

Actually, it is not a stupid question at all. There are a number of nuances which make it tricky to give a general answer. As @collects82 has mentioned, rare in terms of absolute quantity may not mean expensive.

ironhorse is right.

Their scale is based on how many members have a coin. I know for a fact that if you have a coin and no one else has it, the rarity is 97, then 2 members is 95 and 3 is 93.

The commonest coins are 3 on their scale and my guess is at least 10k members of numista (It has over 100k members, but not all are active and not all are going to share their coin holdings on line).

They had a thread on the most common coins and the US Memorial cent, Rosie dime, French 1 Franc and coins like that came first. French coins feature a lot as the site is essentially French, but with a large English language part.

Amen to that.

Its a matter of supply and demand. There are some political tokens that there are a handful out there, but there are only 3 people that care, so they are available. US patterns are mostly rare, but all the collectors of it are dead, so you can buy them.

Pop reports are great, except they dont account for the same coin being submitted multiple times. For most coins, I dont think anyone truly knows how many are left out there, but they can come up with an educated guess.

"Rare" is a relative term. Each country has its own scale of rarity. Here in Australia, for example, Australasian tradesman tokens are graded on a scale of 1 to 10, originally used by the early cataloguers of the series, where R1 is most common and R10 is overwhelmingly rare (unique or almost unique). But, since even the most common of these tokens have survival rates less than 10,000, all of them would be considered "rare" when compared to, say, American coins.

It's perhaps best to consider "rare" as a qualitative term, rather than quantitative. Since rarity is only loosely tied to value, it matters less than it ought to.

"Current population" is a semi-quantifiable equivalent of "rarity". Current population = original mintage X survival factor. The original mintage is usually knowable (except for mediaeval and ancient coins, for which few if any records remain). The survival factor - essentially, the probability that a coin will survive being melted down, buried, locked away in museums, lost at sea or otherwise become unavailable to collectors - usually has to be guessed at, as reliable statistics on such things are impossible to collate.

Collectors themselves are part of the uncertainty, as a coin in a private collection is "unavailable", but only temporarily. Investors are a much more predictable bunch, but the proportion of collectors to investors in any given coin series is also difficult to determine. There's also a continuum between "collector" and "investor", where someone is "a bit of both".

Coins generally become "more rare" over time as their numbers gradually dwindle due to attrition, though occasionally some event can make coins become "less rare". The release of the GSA hoard of Morgan dollars are perhaps the best-known event that created a sudden decrease in rarity for certain date-mintmark combinations in that series. It should also be noted that this "attrition rate" is different for each coin. "COmmon" comins become comparatively scarcer over time, as they are more likely to be discarded and melted than a coin known to be scarce.