Something can be rare, but not worth anything. I have a 4 of clubs with my signature on it. It's rare. One of a kind actually. Worth absolutely nothing.
The elementary level of price determination is supply and demand. If demand for a product increases, the more people will be willing to pay for it. If the supply of a product increases, and demand stays the same, the price will go down because the quantity supplied is greater than the quantity demanded.
Another way to price something is to use the various costing methods that you would learn in a managerial accounting class. I won't get into that, but if you find that interesting....you're rare...
This^. Rare does not necessarily mean valuable.
Rare: (of a thing) not found in large numbers and consequently of interest or value.
I have a one-of-a-kind deck. Custom made for me by a family member. You wouldn't pay me $0.10 for it, though it is among the rarest decks on the planet. On the flip side, I have Gold Monarchs. Multiple of them. They exist in MUCH higher quantities than my one-of-a-kind deck. Much less rare, but much more valuable.
Value is a function of rarity, demand, age, condition, history, etc.
My one-of-a-kind deck? Rarity is extremely high. Demand is almost zero. Age is 6 or so years, not super old. Condition is perfect. History is near meaningless to most people. This all works out to a value of nearly zero. You would (and should) pay more for a deck of Bicycle cards.
The Pre-Release Monarch deck (gold ink on blue, not the Gold Monarchs referenced above)? Rarity is very, very high (only 144 decks). Demand is crazy high, as many people want complete Monarch collections. Age is older than my OOAK deck. Condition varies, but affects the value. History is important in the card collecting world, as it is the start of one of the best selling custom playing card lines in existence. This works out to a value of, well, a lot. You don't see them for sale often so I don't know what they go for these days, but THAT is a deck that will fetch a pretty penny on the open market. The value is only in part due to the rarity.
// L