This all relates to the issue of how much any given mineral is worth. Though rarity, and especially beauty are usually big factors, major exceptions exist. Micromounts displaying magnificent views of an enormous variety of minerals, both rare and common, can often be had for just a few dollars because they're tiny and a microscope is necessary to appreciate them. Many other extremely rare minerals, regardless of size, occasionally go for less money if they are ugly or if the market for them is limited enough. In the March-April, 2009 edition of Mineralogical Record, Rock Currier described it this way:
If the mineral is so rare that only two or three specimens were ever produced, most collectors may never be aware of them, and thus a market value for them cannot establish itself. Rather than pay a high price, the average collector will be merely puzzled by the specimen and view it as a curiosity rather than a valuable rarity. After all, if this is a highly desirable specimen, why don't their friends or local museums have one and why haven't they seen one in pictures? An absence of knowledge discourages purchasing. They have no yardstick by which to measure the desirability of the specimen.
As one who loves, collects, and acquires both rare minerals and micromounts, I'm often elated with this state of affairs. When selling them, however, the going sometimes gets tough. On both ends, the bottom line is the price for which one is willing to part with a mineral and what an able, willing, and available buyer who wants it will pay. The number of both sellers and buyers for extremely rare minerals is relatively limited. Knowledge and experience in this niche are key.