But that would also mean more CPUs, PSUs, RAM, HDDs, etc. which eventually would likely total more than the one system build.
I haven't run the numbers but I'm almost positive it isn't justifiable because you can get 6 slots for less then half. That means your paying 280 for 2 extra slots which is definitely more then all the other parts required to build a server. Also if you argue the psu should be included keep in mind that with 8 gpu's you would most likely need 1500+ watt psu and those things are fucking expensive.