Discus are in my opinion the most beautiful freshwater fish.  The most prominent species of Discus (Symphysodon Aequifasciatus) has more color and pattern variations of any other fish, with possible the exception being Guppies.

That is why they are in High Demand.

They are notoriously in hard to breed, with the fry needing to eat slime off the side of their parents for the two weeks.  Unfortunately, the parents almost always eat them.  If you try to raise them artificially, it is extremely labor intensive for the first two weeks, requiring you to do water changes and feeding the fry every four hours.  They also grow much slower than other Cichlids that are similar, such as Angelfish.

The reason that they are so expensive is because they are in high demand and in low supply as there are less of them than is necessary to drive the price down.