If people are not willing to buy them at a very high price, their prices would naturally go down.
Old vanities are very expensive because they are desired, and their scarcity is a significant factor on why they are expensive.
Very rare items are still selling at high prices because there are only a few of them that exist, and the cost of obtaining them is high as well.
White rabbit plush and temporal capes are ultra rare pulls from lucky reds.
If they are cheaper, then the price of lucky reds would be cheaper too. But each envelope is expensive, and you are not even guaranteed to pull the ultra rare vanity from them even if you open lots of them.