To make it stackable it costs 61 USD.
Also only 7 slots in the backside, so not 100% useful for setups with more than 2 cards (if you leave an empty slot of space between cards)
In the end I would prefer a case where you can mount GPUs via extenders on a different level than the mainboards - bonus points for extenders already included.http://www.cablesaurus.com/
sells such cases, but at a quite high rate for my taste. I would in the end just build an IKEA Lack rack and do the missing stuff like a bar for GPUs to rest on and a mainboard tray in one afternoon...