Coin collecting is a hobby of questions and answers
How much is any given coin worth? Some coins have their value determined by the spot price of gold or silver. I have seen, in the past year, slabbed MS64 US $20 gold pieces being sold at only 6.5% over their gold bullion value, not counting the slab! A Roman denarius has about $3 worth of silver but is worth a lot more than $3. Why? What motivates a "numismatic" premium?
Is the coin at the upper right worth a little or a lot? (This particular coin is considered near the end of this page).
What is the value of a coin? There are at least two different meanings of "value":
1) typical commercial value (at auction or at a fixed-price source),
2) personal value to you (because of your personal knowledge about it, your desire to own it in light of your collecting goals, and your finances).
The commercial value, retail and wholesale, is, of course, related to supply and demand. Demand is the result of the personal values assigned to it by many collectors. Dealers must emphasize commercial value in order to make a living. However, you, as a collector, pick and choose what you like enough to actually buy. It is important to realize you choose not to buy almost all coins on offer. Almost all coins have personal value to you which is less than their cost, so you don't buy them. This web page asks you to think about what makes the value of an ancient coin high enough--to you--to make it worth more than its cost.
Of course, personal value to you and commercial value are in a feedback loop. The commercial value is determined by personal values of many collectors which are influenced by commercial values. For example, if you think a particular coin which is offered at $50 is worth $100 commercially, you might buy it just because it is a "good deal." For another example, a beginner might be willing to pay, say, $300 for a lifetime portrait piece of Julius Caesar. However, as he learns more about its commercial value (which is higher than $300), he might significantly increase his personal value for that type (or he might decide he won't buy one). Your judgement of value is, of course, influenced by the market in which you participate.
So, undoubtably, commercial value is very important. This webpage emphasizes how "personal value" is related to the questions that you can answer about the coin.
Goals. You can't buy them all, so you will develop criteria for buying. Maybe your first ancient coin was bought merely because "It's so old!" However, eventually you will develop collecting goals that help you describe to yourself what you seek. I started a CoinTalk thread on value and got numerous interesting responses about collecting goals, some of which I reproduce (with permission) below. The number of potential goals is huge (Some are discussed here). "Personal value" is a concept that can help you select a goal that makes your coins worth--to you--more than they cost.
Contributions to personal value. You get a small jolt of pleasure when you know the answer to a question. (That makes it fun to follow Jeopardy.) There are very many questions that can be asked and answered about ancient coins, and a coin of yours will provide pleasure as you answer the questions that come to mind. Who is the ruler? When was it issued? What is the metal and denomination? What is its commercial value? Is the coin very nice for the type? Is it related to some historical event?
Each answer you know adds a small positive feeling that increases the personal value of the coin to you, but not to someone who doesn't know the answer, or possibly even the question! Show a Roman denarius to a US-coin collector and he will note it is small and silver and he will inspect it for wear (because that matters a lot for US coins). He might wonder what he is supposed to be looking at and how he is supposed to make sense of it. He will wonder about its commercial value. He might not know the standard questions an experienced ancient-coin collector would ask and answer. It is likely there is so much about ancient coins that he doesn't know that he can't begin to appreciate the coin "for what it is." You need a trained mind to appreciate the numismatic premium of a coin.
That explains one of the reasons a US-coin collector will probably not buy a $100-dollar ancient coin, even for $50, because he doesn't know enough to appreciate it. He doesn't know the questions and answers that give it personal value.
There are reasons why a coin type was minted at a particular time with its particular design. If you don't know the reasons, those bits of pleasure are denied to you and the coin is worth less to you.
It would be nice if we knew "everything" about our coins. Some things we might know are more interesting than others, but every little bit adds up and can be appreciated again and again whenever you look at the coin. As you become more experienced with ancient coins, your list of questions and answers gets longer and your potential for enjoying the coin increases.
The basic questions:
1) Who? (It's a coin of a Roman emperor! It origin. Its issuing authority--Greek city, king, emperor, or provincial city.)
2) What? (Denomination--currency unit--and metal.)
3) When? (It's old! How old is it? When was it minted? The precise year is sometimes obtainable, but often a date range must suffice.)
4) Where? (Where was the mint? Where did the type circulate?)
5) Why? (What is its story? Why was it issued? This question has many levels to be discussed more below.)
6) How much is it worth? (This webpage discusses how this question has many levels.)
Additional numismatic questions:
What is a reference-work ID number for it?
A key for personal value is:
Does it fit your collecting goals? Does it fit some theme (topic) or mini-theme you would like to collect? (Collecting goals are discussed more below.) Do you "need" it for your collection? [US-coin collectors often seek "sets" such as all the types of Indian Head cents. The set is well-defined and finite. Eventually they will "need" the ones they don't have. Ancient coins are different. There are not many well-defined "sets" in ancient coins. "Themes" is a word that better describes how ancient-coin collectors collect.]
Is it artistically attractive?
What is its "condition"? (Learn not to conflate "grade" and "condition". This is an issue with many considerations. I have a webpage on it.)
What factors influence the condition and how important are they? (Wear, surfaces, patina, cleaning, corrosion, nicks and dings, test cuts, tooling, etc.)
Is it common or rare?
How does the condition compare to other coins of the type? (Is it good "for type"?)
Is it genuine? (How can you be sure it is not fake? How can you detect fakes?)
(The list goes on, but that's enough for now.)
Why? Why was your coin issued with that design at that time? Maybe there was some event that provoked it. That would be interesting to know. If it is an imperial Roman coin it names the emperor (which is interesting in itself. There is a lot to know about each emperor). You can get pleasure from answering the question "What does the type refer to?" Maybe you don't know, but often a short search will find the answer. Maybe you can find out if you read more widely. (Some of my collecting friends began knowing nothing about ancient coins and have advanced to the published-research stage of answering such questions.) If you do find out an answer, every time you think about owning the coin you get a bit more pleasure than without that knowledge. The coin's value is greater to you.
Your ancient-coin would not exist without a reason. That reason may be related to something in the historical record. If so, the coin is "better" (can provide more enjoyment) than types which cannot be related to history. You can read about Trajan's wars in Dacia (modern Romania and Moldova, with bits of other countries) which motivated this bragging type:
Roman emperor Trajan who reigned 98-117 AD.
A silver denarius celebrating his victory over Dacia.
What is on the coin? (You will appreciate it far more if you can answer this question.)
What is the obverse legend and what does it mean? (It has his name "TRAIANO" at 7:00 and includes "DAC" at 1:00 abbreviating his new title "Dacicus" = Victor over Dacia.) But it includes more. Can you answer what is there and why it is there?
The reverse has a mourning Dacia (personified) seated on a pile of arms from the war.
DAC CAP abbreviates Dacia Capta (captured).
I have a webpage on Trajan's historical types.