After the recent difficulty increase 8 GH/S is down to about .05 BTC/day.
In related news...
So a upgraded Jally after the price hike is .05 BTC/day (71 day ROI starting today)
a 400 GH/S Bitfury unit is 2.5 BTC/day (28 day ROI)
a BFL minirig 500 is 3.1 BTC/Day (63 day ROI starting today)
an eruptor is .0002 BTC/Day (about 2000 days to ROI?)
a Monarch is ~3.75 BTC/day (11 day ROI if it started running now).
Difficulty changes about every 14 days. Assume an ~20% increase each time, then see what numbers you come up with...
Example:
400GH/s BitFury... 28 days?
Assume difficulty changes biweekly, one after the first week, and one after the third..etc..:
After...
First week 25% is paid for (assuming your ROI of 28 days with constant difficulty correctly equals 100%).
Second week difficulty goes up 20%. So, you pay off 25% * 80% = 20% more (total 45% paid)
Third week: same difficulty.... +20% more = 61% paid
Fourth week difficulty goes up 20%. So, you pay off 20% * 80% = 16% more (total 77% paid)
Fifth week: same difficulty.... 16% + 77% = 93%
Sixth week difficulty goes up 20%. So, you pay 16% * 80% = 12.8% more (total 105.8%).
>>You actually broken even 7/12.8ths into the sixth week or 3.82 days into the sixth week.
5 * 7 + 3.82 = 38.82 days to ROI. A (10.82/28 * 100)%, or 38.6% increase in ROI time.
Quite different when you factor in difficulty.
Caveats: Assumptions made were that difficulty increases by 20% each time. However, this can change even to the negative.
But seeing how BFL is shipping still, this is not likely to go negative. And the assumption of minimal system down time.
Edit: If power costs were not factored in the original, this would take even longer... perhaps another four days to a week.
