Asssume that such 1-month attack happens, its cost will be about $4,250,952 ($590,410*24 hours*30 days).
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.
I would argue you can not calculate it that way. Simply because access to the amount of gear/hashrate is not possible for anyone to do.
The only way to attack the network at a 51% rate would be to divert the 3 largest pools via hijacking of the hashrate. If this was possible miners would shift to other pools. The attacker would need to be able at access pools at will to divert the hash to his address.
Lessor networks can be attacked.
BCH
BSV could be hit for a 51% attack not that hard
viabtc.com could do it on the sneak. viabtc has 9eh mining at btc they could simply pay the miners out of pocket and divert the the hash to attack with bsv or bch.