Presently at Slush I get approx. 0.002BTC@~600MH/s at each block (averaged). We find around 16-18 blocks per day.
Assuming I were to pay 2.2BTC for this usb 'block eruptor' I would be looking at:
2.2 / 0.001 / 17 = 130 days to break-even, assuming network difficulty remains as-is, which it WILL NOT.
Because estimates of rate of difficulty increase vary wildly I hesitate to _speculate_ further, but it seems to me that there is a possibility this buy will not pay for itself in a reasonable time frame.
There are two more things to consider - let me use some examples:
- I have 5850 cards which use about 80-85W to produce roughly 300MH (or say it the same as that USB key). Here is the USB key advantage - it uses 0.5W.
- I got my cards at $120-150 each. Some are on dedicated mining rigs, some are used for gaming. At the end of the day I can sell those and recoup some of my investment.
My observations about payout are similar to mrm0's - with one rig at 900MH/s my average is about B 0.035-0.04/day (depending on luck). Or it will pay itself off in about 160-200 days (at current difficulty levels).
A GPU card (priced let's say 1.1BTC at current levels) making the same 300MH/sec minus price of electricity (2KW@$0.15=$0.3 or B0.0025) will bring you B 0.01/day or given the same presumptions will pay itself off in 100 days.
So, after the same 200 days each of those cards will pay itself and bring me about $120-150 profit + I can sell the cards for let's say $75 = total profit $200/card.
I'm over-simplifying it but you get the idea.
Bottom line is - if you're just now entering the mining world it is probably not worth the money as there are better alternatives.
EDIT: See this site for calculating the break-even duration: https://bitclockers.com/calc