Current global hashing power: 1.97E+12 Hashes / Second (

source)

That puts the odds of any given 1 Mhash/sec winning a block at 0.0000506%. Over the course of "today" (the next 144 blocks) it has a 0.00729% of finding a block.

The current growth in difficulty suggests an expected growth in global hashing power at 3.5% per day. That means that tomorrow, the chance per block will be 0.0000489%, and overall 0.00705%.

Assuming this rate of growth, the limit (sum of the series over an infinite lifetime) is 0.215% chance of finding a block.

So, a GPU, or whatever, with 300 Mhash/sec of power has about a 65% chance of finding a block between now and the end of time. The expected payout is thus 32.34 BTC, or about $226 at $7/BTC.

Thus, with my pessimistic assumptions, you should not spend more than 75 cents per Mhash/sec, which is still possible, but getting harder.

Note also that I'm assuming zero operating expenses (free power and cooling).