Element essence crystals are, of course, something out of reach for ordinary people.
But for Transcendents, actually, they're not.
The essence of an element essence crystal is nothing more than elemental power condensed into a substantial form. Transcendents have too many methods to create such things; a rather crude one is to summon elemental creatures and then kill them. The corpses of the elemental creatures naturally form these crystals, especially the Earth Elementals. Besides, some capable individuals can even condense elemental crystals themselves—this is also a way many of them make money.
Of course, among all the elements, aether element crystals are indeed the most expensive. One reason is their scarcity, and another is because it's difficult to condense aether elements into crystalline form. An aether element crystal of the same size and quality often costs three to five times more than the cheapest Earth elemental crystals.
But Sherry was quite wealthy at the moment.