Your ROI is all wrong. You are wanting to get BTC for BTC. You said that right now you get 4.2BTC/day and it would take you 20 days to get that back. Your looking to get back 80BTC that equates to at todays rate of $230/btc or $18,400 when you paid 80BTC at $16 you paid $1,280.
BTC is worth what it will buy and a 1 BTC today will buy a lot more than 1 BTC from 6 months ago. So you can't use BTC for BTC as your ROI. If the price of BTC stayed relatively the same then you could. So you have to use something else to determine your ROI say USD which does not fluctuate as much.
If you not happy with that ROI I would be more than happy to take the unit off your hands
BTC is worth what it will buy and a 1 BTC today will buy a lot more than 1 BTC from 6 months ago. So you can't use BTC for BTC as your ROI. If the price of BTC stayed relatively the same then you could. So you have to use something else to determine your ROI say USD which does not fluctuate as much.
If you not happy with that ROI I would be more than happy to take the unit off your hands
Can any explain to me why we will not see a strong correlation between difficulty and exchange price to USD/EU etc.?