One thing I wanted to add on here is that you have to consider opportunity costs. So if you have 3 high powered Radeon cards mining, it could still be technically profitable to mine with btc at $1.00 and difficulty at 1,700,000, but you need to consider the value of selling those cards on ebay and buying a treasury bond or some other investment. There is also the depreciation on the cards themselves, as gaming hardware doesn't hold up value very well over time.