Here you go...
tomorrow exchange rate of bitcoin goes to 0 and stays there forever. You've just lost 100% of your bitcoin investments while me and my investors will have some computer hardware and a working supercomputer which coluld be sold or may be used to mine namecoins or bitcoin 2.0 or towncoins or googlecoins or visacoins or ciacoins or even fedcoins or carotcoins to be rented out to GCHQ or something else.
Goodbye.
You didn't think it would be that easy...
It's time to look at the number once again...
Thats where the answers will be found...
Not in peoples lofty dreams...
So Im going to assume that if bitcoins fall to 0 overnight, the value of the equipment will probably fall to something like 50%. So your investors are risking:
1000 investment + 100 monthly payment - 500*65% value of investment after BTC becomes worthless = 775GBP
Now what I am going to do is buy coins today for 775GBP.
That would equal about 150 coins.
Now every month I will spend the 100GBP buying coins. With average difficulty level of 1M, I should be able to pick up at least on average 10 coins per month. After a year I have 260 coins this way and risked exactly the same as you.
You might argue, but what if price goes up a lot... Well then difficulty will do the same, so it will be worse for you because I get most of my coins in the beginning... Even giving you the very favorable assumption of average 1M difficulty over the next year, which I think most would agree is kinda unrealistic...
The end of the story is:
Your clients risk: 775GBP to gain 237 Bitcoins.
Buying bitcoins risk: 775GBP to gain 260 Bitcoins.
And that my friend is why its called a risk/reward profile and why you just don't look at the risk or reward in isolation.
Investing in bitcoin mining today is questionable...
Investing in bitcoin mining today keeping only 65% is outright terrible...
If its a bad investment...
There is no way you can play around with numbers to make it good...
Its the beauty of the game...