.015 BTC per day should ROI in under a 100 days. I already had the PSU's from S1's so no additional costs there. 0.75 BTC is the hairy edge IMO.
Except its not .015 / day. At 15%, starting July 15 its ~ .01164
These are very unlikely to ROI @ .75 BTC each
Really and you know what the diff will be how? Network has been under 6blks/ hour for 23 hours how does that equate to a 20% rise again?