How can you mine 25(!) BTC with equipment worth ~500 USD within 1 month?! This is BAD advice!
What do you mean? I can just go out and buy parts from stores around my home. Worst case, I need to wait 2 days for shipment. Wtf 1 month?!
You cut the quote... maybe it's easier understandable this way.
About the recent babble of "Difficulty follows price!!!!!111" (+add some skewed excel-charts where 99% of data points are crammed in one corner to the mix):
Unfortunately this chart seems to be discontinued - I'll still link it as a warning though:http://bitcoin.atspace.com/income.html
Please note the logarithmic scale!
If price increases as you say, you're better off cutting difficulty (+ noise + electricity costs + work hours + heat...) from the equation and just keeping BTC, as they will increase each difficulty increase anyways in value as well!
Mining would only pay off if you end up with more BTC by mining after some time. With current increases, it doesn't look like you will mine 25 BTC within the lifetime of a card that costs 25 BTC right now. Also you have to add constant costs like electricity + work to that equation!
Your graph assumes someone cashes out daily. Although it's a convenient argument for people saying "Zomg coin speculation means you shoulda just bought coins!" it's not true. There is nothing to the argument.
Here's a simple scenario, but not oversimplified:
25BTC = $475 USD today. Cash out, buy 2 5870s from ebay at inflated prices, wait 4 days for shipping. Put it in existing rig.
You now have 850Mhash/second, and about 6 days of this difficulty left, give or take.
Assuming low variance -- That's 9 BTC before jump.
If we assume a 30% difficulty increase thats roughly 11.16 more BTC (20 total) after 10 days.
Assume another 30% difficulty increase that's roughly 8.9 BTC after 10 more days.
30 days total: 29.06 BTC. You now have more BTC than you started with, the new price of BTC is irrelevant, and you also have $400 worth of video cards. The reason I say price of BTC is irrelevant is that 4 BTC = $75, far less than the cost of electricity. If BTC value is lower you would have lost money on holding BTC. IF BTC value is higher you made more money than you would have anyway.
If you change difficulty to say 40% increases you get 9 + 10.8 + 8.3 = 28, still more
50%: 9 + 10 + 6.7 = 25.7
This is about the limit of the 3 difficulty window return scenario. 60% requires 4 difficulty jumps, and is incredibly unlikely as a scenario. As it would be a 2.3M difficulty after 40 days month, or a 400% increase in difficulty / hashing power (16.something THash/second).
I'm not suggesting OP do this, but I would like to point out the fallaciousness of the idea that it is always better to buy coins rather than mine and mining will never pay for itself. In fact right now is an auspicious mining time. It can change, but that is where we are now. The scenario for buying coins is huge difficulty increases coupled with only moderate increases in coin value. This happened before and can happen again.