How much would it cost in U.S. dollars to brute force a 256 bit key in a year?
The average cost of electricity in the US is $\$0.12$ per kWh. For a single server, I'll use 3741 kWh annually as an estimate. That would be about $\$450$ per year for one machine. 

Let's say you can do $10^{14}$ decryptions per second. That is $3.15\times 10^{21}$ decrypts per year for one machine. You need to do (on average) $2^{255}$ decryptions in a year, so you would need $\frac{2^{255}}{3.15\times 10^{21}} \approx 1.84\times 10^{55}$ machines. To figure your cost you would multiply that by $\$450$ and get about $\$8\times 10^{57}$ or 8 octodecillion dollars. Gross world product, or GWP, is about $63\times 10^{12}$, so brute-forcing a 256-bit key would cost about $10^{44}$ times the GWP.

You can follow similar math to get the cost of brute forcing a 128-bit key.



Note:

I am completely ignoring hardware costs, maintenance, etc. The estimate above is for electricity only. We can take a hint from the NSA on this. You'd be a lot better off hiring a few thousand mathematicians and have them work on breaking the cipher as opposed to trying to brute-force it.