Nice calculation. certainly it is not worth building a machine for bitcoin creation. However if you already have this computing power sitting idly then you should calculate only the costs of the extra electricity that you need to consume to generate a block. Has anybody done this?
If you want to build a bitcoin generation machine you are probably better off buying a FPGA board and designing a dedicated hash calculating hardware monster.

Interesting note: If one puts 4100000 hash/sec in the calculator then you get 10 minutes average.
10 minutes is the actual target average block production rate.
This means that currently the the total computing power working on solving hashes is 4.1 Mhash/s
or about 900 quad cores. or 3-4 thousand laptops. Pretty impressive!

This means that there are some people who think it is interesting to produce bitcoin.
Who knows why, I stopped several difficulties ago.