Will it pay off?

Looking at the graphs at BitcoinDifficulty.com, it looks like difficulty has increased quite linearly by 8 000 000 in the past 3 months, that's about 1.3 million in each 2 week period.

At 336 MH/s, we would do about 406 TH (Tera-hashes) per 2 weeks.

Here's a table of how much we would make if we started today:

Weeks	Difficulty	BTC/2w	Cumulative BTC
2	12 153 411	0,195	0.195
4	14 820 077	0,160	0.354
6	16 153 410	0,146	0.501
8	17 486 743	0,135	0.636
...
48	44 153 403	0,054	2.239
50	45 486 736	0,052	2.291
52	46 820 069	0,051	2.342

So, sure, after a year we'd have covered the cost, barely. If we'd start mining in a month rather than now, we'd just about covered the cost in one year. At that point we'd be making about 50 mBTC per two week, with the amout decreasing by about 3 mBTC per month.

In this model I have not subtracted the cost of electricity, which I believe for me would be about 1 mBTC per 2 weeks.

Might be a fun thing to give mining a try, but it doesn't look like a gold mine to me.

Apologies if this calculation has already been made, I didn't trawl the entire thread.