The proper business term for that is Supply and Demand. The inverse relationship of those, to be precise. Supply and demand are an inverse ratio, the less supply you have, the more demand you have. Something that is one of a kind has incredibly high demand, since you won't be able to acquire another one. Then you can raise the price really high. conversely, if you have too much of one thing, you can sell it to more ponies, but you will make less money per customer.

20 bits per dress is when you have a shit ton of dresses and you need to get rid of all ten million of them.

that is all.