What Is the ‘Cost of Attack’ and How Is It Calculated?

The 'cost of attack' is the total expense incurred by an attacker to successfully execute a 51% attack, primarily to profit from double-spending. For PoW, it's typically calculated as the cost to rent or acquire 51% of the network's hash rate for the required duration, plus the opportunity cost of not mining legitimately.

For PoS, it's the cost to acquire 51% of the coin supply plus the risk of losing it through slashing.