"How is the attack cost calculated? Using the prices NiceHash lists for different algorithms we are able to calculate how much it would cost to rent enough hashing power to match the current network hashing power for an hour. Nicehash does not have enough hashing power for most larger coins, so we also calculated what percentage of the needed hashing power is available from Nicehash.

Note that the attack cost does not include the block rewards that the miner will receive for mining. In some cases this can be quite significant, and reduce the attack cost by up to 80%."