If a 'normal' Clack is usually sold for 70-100$ or even more, an ultra rare Clack could be for sure 4 times more valuable and hold it's price. So 400$ is not that crazy, if it's very rare. It's all about demand and supply, the only way the prices could go down is lower or almost no demand and disinterest.
[EDIT] Forgot what i wanted to post: sleepless night, 3AM, looking for Clacks to be sold. Even one 3D CC was in the classifieds this night, but i was too slow.