Rarity is something that raises the eyebrows of most collectors. But it becomes somewhat less meaningful for ancient coins. It is easy to see why a modern coin will be worth a lot if there were few minted, or few remain in existence, given the large number of collectors who would all eventually like to have such a coin to fill that last hole in their ’modern’ collection. This will drive the price up. But because ancient coins are collected in a
different way, there may be no ’last hole’ to fill in most of the time. As discussed above, you decide what you want to collect, which will be different from what others collect and so everyone can get what they want more or less.
There are rare ancient coins of course. Some emperors were not around for long enough to have many coins minted and hence they will be harder to find and cost more. However, unless you are trying to get them all (which is impossible anyway), you need not concern yourself with a coin of Julian I for example. A workshop in some city might have struck very few coins with a certain reverse design for a given emperor, but unless everyone is trying to get all the types from this workshop and emperor, the coin will not necessarily be very expensive. Or, all the coins of some type show one star, but one shows two.
Wow! For a modern coin this would make it basically priceless, but for an ancient coin, it will probably sell for the same price as the one star variety, or not at all because it may be in poor condition. The list of examples is endless. So, while rare ancient coins do exist, it will take you a long time to learn which ones are really rare and why, and then you can ask yourself; do I need this coin? If many people say no, then the value will not be high. Finally, a nice coin of Nero will cost you a lot, not because it is rare, but because it is highly desirable.
Sellers of ancient coins will often use terms such as common, uncommon, scarce, rare or very rare to describe a coin. Just because someone describes a coin as rare does not mean that it actually is. Unfortunately, some sellers will attach this label to ancient coins to attract novice collectors who do not know better. Does the person explain why the coin is rare? Does he give it an RIC number and explain the variety?
If you are new to collecting ancients, you should not jump on a coin just because is it described as rare, especially if you are not familiar with it. Scarce is a more accurate description I find, because less sellers would use it to attract a buyer. Eventually, if you do build up a collection and are aware of many coins from catalogs and eBay, you will be able to tell yourself if a coin is uncommon, scare or even rare. Also, truly rare coins (that are desirable) will not sell for much less than $100, and often for much more. Hence, a coin of Constantine described as rare with an asking price of just a few bucks is very unlikely to be so. There are sellers on eBay who claim that their coin is rare, or they have only seen one in 10 years, but there is another one just like it being offered on eBay by someone else!
So, to summarize. Buy coins that you find interesting and that appeal to you. No two coins of the same type will be exactly the same; choose the one that you think is a better value for your money. Look around to see if a nicer looking coin might be also affordable if that is what you really want, and hence save yourself the trouble of replacing it later or regretting that you bought it in the first place. Do not worry about rarity too much!
If you show any of your ancient coins to a non-collector or a modern coin collector, they will be totally amazed that you have something that old and will think that they are all very rare (or fake!). Keep in mind that many Roman coins are not rare.
To give you an idea, quoting Van Meter’s Handbook of Roman Imperial Coins ”…scholars estimate that the mint [of Rome] was striking around 400,000 denarii per month from 85-87 AD, but then production surged to over a million per month the next year, and peaked at 2.5 million per month in 92 AD.” This is far more than many early US coins for example, which are only 150-200 years old.
Yes, many were lost to time, but many still remain, and remember, there are fewer collectors of ancients than of modern. Over time, as you get more coins, some of them will be more scarce than others, and you will have a sense of satisfaction owning them, but to another collector who has a different interest, these coins would not be worth the same as they are to you.