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).