You can also get more effeciency by using 3rd part PCI-E Extenders and plugging in more gpus in 1 system unit. It will lower down your overhead cost on additional system unit. With this I used a 1x pci-e to 4slot 16x pcie extender. It means that if you have 3 pci slots on your motherboard you can host 12 gpu in 1 system unit. And preferably 2x 850w psu to power the 12 gpus.
Below is the PCI extender I used in this setup.
RE: Sapphire Pulse Radeon RX 550... $80 bucks for 450h/s on cryptonote. Bargain for the beginner on alt coins.