> I get how block rewards bring the net cost down, but where does the 80% number come from?
It's a rough number - In the case of Bitcoin, the miner receives 12.5 BTC (+ transaction fees that I will ignore for now) per block, with a block time of 10 minutes, so they will receive 75BTC per hour, or $544,275. In this case it wouldn't be possible to actually rent anywhere near this much hash power at this price, but for smaller coins it would be possible to do so.
> In some cases this can be quite significant, and reduce the attack cost by up to 80%.
I get how block rewards bring the net cost down, but where does the 80% number come from?