What's that attack cost supposed to represent? Just the electricity?
Seems like a misleading comparison, to get into a position to be able to do this you'd need significant investments in specialized hardware for the likes of Bitcoin and Ethereum. I've seen estimates of multiple billions of USD. And keep in mind that should the attack be discovered, which with coins running on open ledgers seems likely sooner rather than later, the price is going to tank, trust in Bitcoin will be broken and your special-purpose hardware will likely massively lose value. You'd have successfully destroyed billions of your own money.
On the other hand, the real cost of attacking some smaller coins may be even lower than that, because botnets are free or the electricity may simply be stolen, which happens a lot and can easily be done in less developed countries where the utility companies don't have sophisticated meters keeping track of where it all goes in the neighborhoods.