Silicon nitride, Si3N4, and Zirconia, ZrO2, are perhaps the most common ceramics used in bearings, but there are many ceramics that would work well in this application. There are also different grades of both hybrid and full-ceramic bearings, based on smoothness and uniformity of the balls and races, and a ceramic bearing also has an ABEC rating reflecting these characteristics.
There are also different levels of seals available. All of these variations result in differing prices. And of course, price is also a function of supply and demand, and the demand among cyclists has been quite high lately.