That's still a ROI of 10 months for cards bought at $250
Your calculation is based on gpu only, system is expensive, you need to put the system cost too, motherboard around $150, psu around $200, ssd around $80, cpu around $40, memory 8GB around $80, total system cost without gpu around $550 and that is average price of a system, most countries you pay 50% more on the $550 value. Which means $800 without any gpu, now you place 5 gpus which is also standard per motherboard, your roi is not 5 gpus, is 8 gpus cause of the system cost, $800 is 3 gpus based on your $250. So total to roi is $2000 plus electricity is around $0.20, now let's see, profit per day is $3.40, $2000 divided per $3.40 = 590 days to roi, that is almost 2 years.
You're way overbuilding the base rig...
) 
