Short answer : 256-bit key cracking through exhaustive search is totally out of reach of Mankind (http://security.stackexchange.com/questions/6141/amount-of-simple-operations-that-is-safely-out-of-reach-for-all-humanity/6149#6149
But if you still want to try and use all the dollars in the World (including the dollars which do not exist, such as accumulated debts) and fry the whole planet in the process, you can barely do 1/1000th of an exhaustive key search on 128-bit keys. So this will not happen. And a 256-bit key search is about 340 billions of billions of billions of billions times harder than a 128-bit key search, so don't even think about it.
And from the energy point of view : if we built a Dyson sphere around the sun and captured all of its energy for 32 years, without any loss, we could power a computer to count up to 2^192 in 1 year. But this is even not enough energy to do any calculation, just enough to increment a counter.