As a maker of plashes I have two views, they take a decent amount of time, skill, practice, trial and error, and tools to make depending on the level you're making them.
I can't see ever selling a custom plush for the price build a bear sells them at, just because the price to make them amounts to about as much as they sell them for not even touching the cost of embroidery machine, or hours to hand embroider. But if you look at the quality of most sellers compared to the ones you see at the store they look more realistic, more trust to character, you can get custom OC's that build a bear won't carry and you know that it's hand made.
Although, I could also never see selling or buying one for 400 no matter how obscure the character is (unless it was 6 foot). Yes they are art and artists need to be paid for their art, but sometimes there is just too much for a good thing.