You also have to factor in hardware depreciation costs.. New video cards come out all the time, usually once a year but can be sooner... and also the fact that other technology could come out and disrupt your entire "large scale" operation (ASICs etc). Technology moves very fast - it's going to have to be higher than $.30 per gigahash. At that rate you'll almost certainly go negative, unless you play the markets and make sure to sell your hardware at the right time.