Antminer S3, it seems cheapest mining hardware in the world. BUT it is not.
Assuming you will mine with this device for one year. Because after a year, hardware mines nothing because of diff rise.
Lets calculate the how much it costs..
0.68 BTC for 441 GHS.
340 watt for 441 GHS = (0.34kwh/441ghs)*0.15$/kwh*24*365/(550$/BTC) = 0.00183 btc/ghs in one year energy cost with 0.15 $ / kwh
0.68/441 = 0.00154 btc/ghs comes from order
0.00183 btc/ghs comes energy cost
PSU : lets say, 75$ ----> 75$/441/550$/BTC = 0.00020 btc/ghs
TOTAL COST IN ONE YEAR
0.00154 + 0.00183 + 0.00031 = 0.00368 BTC/ghs = 2.02 $ / ghs @ 550$/btc
Some advise for you, just try cloud mining services with has no fee policy. Just like
http://pbmining.com with prices 0.0029 btc/ghs for five years and with no maintance fees, no pool fees, no any other recurring costs,hardware mining is useless at the moment.