We pretty much have to look at some boutique stores from Japan, China and Russia who have tried to clone the WE tubes. All very expensive. :/
You raise a really good question. As stated, the demand for tubes is very low and no one would make any serious money out of it.
Strangely enough, the price is pretty much fixed at the RRP set by Audeze. I'm not entirely sure if this is because the demand is so high or because Audeze have 'set' the price in their distribution agreement.