A 51% attack will never happen. The miners would never do it because they know that would be the end of the crypto world, and all their investments would turn to dust overnight.
So.... checkmate FUD'ing concern trolls?
Unfortunately, there are margin short options which would allow the attacker to make a boatload of cash if the price falls. However, at the present time, there probably is really not enough liquidity in any of those markets to offset the huge cost. However, with the CME and CBOE on board, that could change in the future...