A flowerhorn is a mixture of several central american cichlids (trimaculatus, synspillum, festae, labitium, cintinellum, etc.....) that over time have taken on the look of the flowerhorn today. There are quite a few disctinctive strains that you can dig around on here and find.
The reason they are so expensive is because of EXTREMELY selective breeding and countless hours spent trying to match the perfect fish. Less than 20% of each batch of fry will turn out looking good. Only about 2% will carry both strong genes from the parents. Therefore all of this hard work only yields a very small amount of perfect fry and they are worth thier weight in gold. In the flowerhorn world you get what you pay for. You usually want to buy fish that are showing color, pearls (depending on the strain), shape, finnage, and kok (pronounce coke and simply is the nuchal hump).
Hope this helps you out. Any more specific questions please post them and someone will help you out!