I've been following this thread with interest so started researching. It looks promising but I found this:
Bitcoins per Block – Each time a mathematical problem is solved, a constant amount of Bitcoins are created. The number of Bitcoins generated per block starts at 50 and is halved every 210,000 blocks (about four years). The current number of Bitcoins awarded per block is 25. However soon enough the
block halving will occur and the reward will be downgraded to only 12.5 Bitcoins.
Bitcoin Difficulty – Since the Bitcoin network is designed to produce a constant amount of Bitcoins every 10 minutes, the difficulty of solving the mathematical problems has to increase in order to adjust to the network’s Hash Rate increase. Basically this means that the more miners that join, the harder it gets to actually mine Bitcoins.
which to me looks as though from the next block halving you're on diminishing returns for the same processor/GPU power.
Or am I reading this wrong?