Ancient Coin Value

I agree with David that eye appeal is how I choose. Well centered, well struck and a nice patina is also important but you have to know a bit about the coin type that you are going after. Some types where not struck well and even struck on small flans where most of the design is missing. This is the case on some of the Eastern empire types so in that case you might not find a coin with a lot of eye appeal but it might be a rare type in any grade. Unless the coin is a very rare type in very high grade I shutter when I see and ancient coin in a slab. They were meant to be handled and locked away behind plastic.

Personally, "gem mint state" and "ancient coin" are mutually exclusive phrases; a coin can be one, or the other, but not both. The grading companies tend not to agree with me on this, of course.

For me, and for many collectors, a key component of value is whether or not the "key details" of a coin are present and clearly visible or readable. Eg. for Roman coins, the most important thing for most collectors is the Emperor's name. For Athenian owls, it's the owl. For Roman Provincial coins, it's the name of the city. And so on.

Coins on which these key details are missing (either off-the flan or worn/corroded away) are much less valued than coins on which these details are clear and sharp.

Some time ago, I bought a coin of one of the rarer Roman emperors - Galba, I believe - on which only the last letter of his actual name was visible. It was much cheaper than otherwise-identical coins of Galba on which the name could be clearly and entirely read.

Of course, it's entirely possible to align your collecting goals so that the things that are "key details" to you are different; if you collect Roman coins by portrait, for example, rathe than name, you can add to your collection fairly cheaply by buying coins that have no name, but excellent portraits.