Just some math for those buying these miners. I was thinking of buying 100 USB Eruptors for .32 BTC each (so 32 BTC in all). That would require 10 hubs so assume another 2 BTC expensed there so 34 BTC on all which is now about 3400 USD.
The latest difficulty increase is in and we went from 26,160,000 to 50,810,000 in 30 days (from 7/14 to 8/13). This is a monthly increase of 94% - completely insane. This will probably continue for a few months and then drop precipitously once all these new 28nm ASICs come online in the first few months of 2014. Assuming a continued 95% increase per month difficulty increases (a likely scenario for the next several months with all the new hardware coming online).
With a USB hub, 10 usb miners plus hub consume 330W/hr. I took my power costs in the US at 0.11 USD per kwh and plugged it in.
Not only will you lose money but at a 95% month over month difficulty increase, these miners will only produce 18.65 BTC (out of 34 BTC invested) in 24 weeks before becoming unprofitable to continue running. A pretty big loss.
In fact, in order to EVER break even on this investment, you have to assume the month over month difficulty increase will drop down from 194% to 30% to ever break even. A very unlikely scenario. If you do not pay for your power (live in a dorm) then the situtation only a little less bleak. You can break even if month over month difficulty increases drops to 35%. Also very unlikely.
Here is where I get my #s:
Historical Difficulty increases:
http://mining.thegenesisblock.com/95% month/month diff increase calculator:
http://btcinvest.net/bitcoin-mining-profit-calculator.php?diff=50810339.048276&dcosts=3400&diff_mincrease=95&blpbtc=25&dhsmhs=33400&btcusd_mincrease=0&btcusd=100.00&dpowcon=330&dleadtime=0&pcost=0.11&calcweeks=25&action=calc30% month/month diff increase calculator:
http://btcinvest.net/bitcoin-mining-profit-calculator.php?diff=50810339.048276&dcosts=3400&diff_mincrease=30&blpbtc=25&dhsmhs=33400&btcusd_mincrease=0&btcusd=100.00&dpowcon=330&dleadtime=0&pcost=0.11&calcweeks=56&action=calcConclusion: Do not buy USB eruptors or the Old Blades if you wish to make a profit. Your only hope of making money mining is to pre-order some vaporware from hashfast, cointerra, or one of the other 28nm newcomers and pray they are not a scam and can deliver on time. It is much safer to simply buy ASICMiner stock than the hardware!
The numbers for a blade are no better. Only will break even if difficulty goes up to only 35% month over month at stock speed. Overclocking brings that down only a little (still over 30% difficulty to break even), since significant more power is used for cooling and overclocking (and you need to pay for a cooling unit upfront)
http://btcinvest.net/bitcoin-mining-profit-calculator.php?diff=50810339.048276&dcosts=1025&diff_mincrease=35&blpbtc=25&dhsmhs=10752&btcusd_mincrease=0&btcusd=100.00&dpowcon=75&dleadtime=0&pcost=0.11&calcweeks=100&action=calcThankyou.