If I bought a Black Arrow Prospero X-3, assuming the difficulty is 10Billion in May, how long could I remain profitable if it ran 24/7? (Energy Cost: .09USD/kWh)
ROI is relative to the currency you used to pay for the miner. It will take a lot longer to get A positive ROI if you paid in BTC compared to USD as the BTC/USD it generally expected to rise. coinwarz.com has useful calculators, I tend to work on $USD/day figures.