What’s the cost of a 51% attack?

On the 3rd of June, ZenCash suffered from a 51% attack where more than $500,000 in double spend transactions was lost. The attacker managed to manipulate blocks which allowed him to double spend on two massive transactions of respectively 13,000 and 6,600 ZEN. According to estimates via the website 51Crypto.app, the attacker only had to spend $30,000 to pull off the 51% attack.