Cryptocurrency researchers from FECAP University in Brazil have shown that it would take only around $1.5 million to attack the ETC network and still pull a nice little profit. With $55 million dollars, you could effectively bankrupt the currency, netting nearly $1 billion in straight profit.If a party that controlled just 2.5% of the Ethereum hash rate switched to ETC, they’d instantly control over 51% of the total network hash rate. The attack wouldn’t even be absurdly expensive. It’d cost what you would earn mining on the ETH network with 2.5% of the hash rate, which equates to roughly 525 ETH, or $318,000.

Source: 51% Percent Attacks: Hacking a $2 Billion Dollar Cryptocurrency for Less Than $1.5 Million – Bitcoinist.com