Its just like mining gold I guess... you have to spend money on mining equipment to dig for the gold just like buying btc mining hardware. You just hope at the end of the day that your mining equipment produces enough gold, or btc, to make it worth your while. The deeper you go into the mountain (difficulty) to get the gold the more equipment you have to buy... Its definitely not a set and forget system. You have to stay ahead of the difficulty curve and continue to reinvest some portion of your earnings to stay ahead of that curve.