Perhaps you haven't seen what the "nearly normal" looking morph does when combined with other morphs? A good example is the crystal gene. Look up a "special" morph then look up a "super special" or "crystal". The almost completely normal looking "special" turns into a completely different animal in combination. A mojave and special makes a crystal.
Or take yellowbellies or enchis. They also look close to normal but when combined with others or even with themselves to make a super(yellowbellies super form = ivory) they make incredible animals.
The newer the gene, the higher the price. When the gene can make incredible combos, the price will be higher. When people discover something awesome that the gene can do, the demand can go up also.
I still have trouble seeing a yellowbelly, it looks like a regular normal. But I know what yellowbelly can do in combinations. So yellowbelly is a desirable gene and worth a lot more money.