Quote from: BitLex on May 20, 2011, 10:15:52 AM

$18.72 - $2.7 == $16.02 per day
16 / 800 == 2% ROI per day

show me an investment opportunity that pays you 2% per day and i'll join it.

This was my initial thinking, but then I started accounting for predicted difficulty increases and it looked less hopeful; clearly I'm new here, so feel free to point out any foolish mistakes, but here's how I was looking at it:
The $16/day figure applies at current difficulty, which should last for another 10 days or so (probably less, looking at the figures on blockexplorer.com), so that nets you $160 total. A guesstimate based on the previous data would optimistically suggest a difficulty jump of 40% (again, quite possibly more), reducing the daily income to (18.72/1.4)-2.7=$10.67. Say the difficulty levels off somewhat, you might make a further $130 or so at that price, followed by another 20% jump, giving a subsequent daily income of (18.72/(1.4*1.2))-2.7=$8.44. Your daily revenue has halved in less than a month, and break-even might take three months total. Investing in more hardware can't help with break-even time, since Mhash/$ remains largely the same, although it will make the absolute potential profits higher once you do pay off the investment (at the price of a higher risk if the investment is not paid off for any reason).

Of course, the key point left out here is the changing value of the bitcoins themselves - the overall trend seems to be that value very approximately tracks difficulty, so if you're confident that the increases will continue to more or less offset the difficulty increases then my 'diminishing returns' theory is totally irrelevant. It also doesn't account for the resale value of the rig, which diminishes, but may be enough to generate an overall profit once mining itself becomes unprofitable.

Basically, I'm not saying that mining is a bad idea, just that it's a moderately high-risk investment which is inextricably linked to the assumption that the bitcoins themselves will continue to increase in value faster than the increase in mining difficulty.

MoonBuggy

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