Pretty interesting, apparantly you can calculate the cost in dollars to perform a 51% attack on proof of work blockchains. Bitcoin costs over 700k per hour at the moment of writing: https://www.crypto51.app
If one should perform such an attack, wouldn’t there be a way to earn more than 700k from it? (Asking strictly for academic purposes ofc 
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Yeah, it is pretty crazy. It seems pretty cheap to attack, but it isn’t as easy as it looks like. To carry out an attack there is a big risk of losing funds if it fails. Also, there are ways for the network to respond to such attacks, such as DOS attacking the attacker, or requiring more transaction confirmations. Lastly, the possible damage of a 51% attack on the network as a whole is not as big as some might think.
But, it is still a security threat to take seriously.
Well I don’t know about DOSing mining servers on a cloud mining service, but assuming it’s technically possible, you would have to react pretty quickly I guess. You wouldn’t want to postpone executing your malicious plan for too long anyway, because keeping up the attack gets pretty expensive pretty fast. You probably would have to do two trades together worth more than 700K in one hour. That seems to me like much, but not impossibly much.
As to what I’ve read it is quite impossible to get the required hashrate in the first place though, there’s not enough supply to go around. But smaller blockchains like the recently attacked Bitcoin Gold are more vulnerable, due to the amounts of available rentable hash power for Bitcoin. Those chains therefor seem to be more at risk. Hopefully it’s balanced out because there’s less to earn there as well, making an attack unprofitable.