Just to point out the risk involved, in order to maintain the current level of return , new mining hardware has to be continually added to keep up with difficulty. This means new investment is regularly needed to purchase more mining hardware, just to maintain an initial rate of return.
It has already been stated that investment in new hardware will always be needed to keep up with difficulty. Which means that a certain percentage of generated BTC will be put into new hardware. Just as BTC will need to be used to pay for the electric bill as well as other expenses.
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A note: the current situation is exactly why I proposed SIN: we've reached a higher difficulty but price of BTC hasn't followed. This means that there is less ability for current miners to expand and less incentive for new miners to join. However, because SIN is still getting investments (as well as still producing a sizable sum of BTC) we can continue expanding. In fact, this week (the week following a huge increase in difficulty) will be the largest expansion this company has seen since the initial 5 rigs. Once again, I would like to thank investors for their faith in this venture. As Bitcoin makes history, so will we.
Investing in a mining company has other benefits than the hopes of profit, if that was the only reason, I would have been mining by myself.