A farm of a billion GPUs, each with a thousand cores sounds quite feasible with today's tech and gets you to trillion-times speedup of brute force. So, feel free to divide by 10^12... still not tractable.
I was in the market for a single GPU about 6 months ago, it shipped in a week and didn't break the bank. Buying a billion GPUs is an entirely different question. I'd look into buying fab equipment and doing a custom architecture. Given the length of the computation we're talking about, even a decade of lead time wouldn't significant. But as I understand it, current lead times are still less than a year.
