Part of it is in the eye of the beholder, but IMO that doesn't justify certain prices...
I truly am just basing it off of how fast you can divide and spread the plant. The more common something is, the cheaper it will be. High demand, low supply = large prices, high supply, low demand = low prices. It's just economics.
There are plenty of really cool Nepenthes that I would love to have such as jamban or jacquelineae (again), but until I can find them in the 20 dollar range, I wont even consider buying them. Because I KNOW that they are fast growing plants and can be cut up or vined relatively quickly. Same thing with D. regia, it's an amazing plant, and it goes for large sums of money. However, I never actually bought one because I KNOW that people are good at propagating it easily, so why would I pay that much for something that somebody could just take a root cutting of in no time?
The rarer and harder the plant is to grow the higher the price will (and should) be, IMO. If the price isn't low? Don't worry I'll just wait it out another couple years when people are selling melody sharks and wacky traps for barely more than a regular VFT. Cup-traps comes to mind...
Now seed-grown edwardsiana... I suppose if I thought people would be breeding these like crazy, I wouldn't care to spend more than 50 bucks. But that just doesn't seem realistic.