The cost of this attack is about 1080BTC for 30 days or 13140BTC for 1 year, plus depreciation and electricity.
It's not plus, it's either/or. If the alternatives you're comparing are attacking or not mining at all, the cost to attack is depreciation+electricity. If the alternatives are attacking or mining normally, the cost to attack is the lost mining revenue.
Let's go back to the reality. Since there is a 4x difficulty adjustment rule, does it mean an attack of this kind won't be able to reverse more than 4 blocks?