How much would it cost in U.S. dollars to brute force a 256 bit key in a year? I am often told that any key can be broken and that it is only a matter of time and resources for any key to be broken. I know that this is technically true. However, I think that there is probably a point where it makes sense to say a key is uncrackable (for example, if it would cost 100 times the world GDP to crack it, it is essentially uncrackable without the help of an advanced alien civilization, etc.).

How much would it cost in U.S. dollars to brute force a 256-bit key using a strong algorithm such as AES or Twofish in a year?

I would also be curious to know what it would cost to crack a 128 bit key in a year.

I am asking this mostly out of curiosity. I do not know very much about cryptography, so please feel free to pick the algorithm of your choice if that matters. I am interested in how one would project the cost (assume you have to buy the hardware but you get to choose what hardware you buy).
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.