> There is absolutely no way to 51% attack a major coin like Bitcoin for as little as $700k an hour. They are extrapolating from Nicehash's mining rental prices

> There is absolutely no way to 51% attack a major coin like Bitcoin for as little as $700k an hour. They are extrapolating from Nicehash's mining rental prices, but Nicehash doesn't have anything like the capacity you'd need. You are correct - the nicehash-able column represents the amount of necessary hash power that is available via nicehash. If it's below 100% the attack cost is also greyed out. Disclaimer: I build crypto51.