Losses occur when coins, mainly born of a fork of the Bitcoin code and with a low hashrate, are listed by the exchanges themselves.
The Mechanism is This
A 51% attack funds an account opened at the exchange. At the same time it forces the network to restore the transaction (as if it had never happened). The exchange in the meantime credits the sum, minutely mischievous, to the account. At this point, whoever perpetrated the attack makes the cryptocurrency trading freeing up.
But the exchange has actually occurred in a loss condition for the exchange of cryptocurrencies, because the original deposit would be missing from its wallet: it was a 51% attack to spend twice the same sum.
Hacker Calculate the Earnings
Before launching an attack, hackers evaluate the most profitable cryptocurrency network. Then they move on to the action adding to the network of the altcoin a quantity of hashrate greater than 51% compared to what is already present.
This specific technique does not damage the regular coin holders, because the "aggressors" actually do the mining in the network accumulating sums of the cryptocurrency itself. Since they have the majority of computational power, they can falsify the transaction by spending it twice.
The technique is to the detriment of small exchange, which to compete in the market of trade in cryptocurrency welcome just the small cryptomete that the large exchange and brokers do not agree to list.
Crypto51 App says How much to Make a 51% Attack
There is even an app, Crypto51, which says how much it could cost to make a 51% attack on a cryptocurrency.
It should be noted that the app takes into account the rental of computational power from NiceHash (the last column shown in the picture below). But you could also build your own mining farm and calculate costs. The app refers to an attack of only one hour.