You've missed a few digits there. Using those numbers, the cost of a 1 month attack would be $425 million. Although you should also factor in that an attacker who manages to overturn a month of blocks would gain 27,000 BTC in block rewards (plus potentially fees if they chose to include other users transactions in their blocks), which at current prices would give them $280 million, so lowering the cost of a month long attack to around $145 million.