Jackson gave me this link before which helped a lot. I am no expert, people like Jackson, David R, Lmt6600, and Matubula are much more knowledgeable and I am sure they will address any specific questions you have.
http://www.plecoclub.org/forum/showthread.php?t=1589&page=83
Think about it this way... to many, most arowanas looks the same, but silver and black fetch very different prices, as well as different varieties of Red and Gold arowanas. The difference in grades of red arowana can mean more than 500% difference in pricing. To most people that are not into arowanas, they can't tell the difference so they can't justify the difference in price.
Forget about the different names, just take a look at this two picture and tell me which will you pay more for.
a
b
Now, put into consideration of supply and demand. You can buy pleco a pretty much year round. Pleco b comes along once every few years, (there will be at least one available soon! not mine) How much more do you think b will cost more than a?
Some will like a better, come will like b better.