The cost to attack a PoS coin is nothing because they can sell the PoS coin after the attack is successful but prior to the market having a chance to react to such attack. In theory an attacker could deposit the PoS coin onto an exchange, sell it for BTC, withdraw the BTC then double spend the deposit onto another exchange and repeat
Wow, thanks for opening my eyes - this seems to be the most competent analysis of the topic I've seen so far.
Thanks.
This is why PoW is resistant to attacks and why, even if one entity controlled 51%+ of the network they would probably not attack it. Anyone who is mining will receive a reward that is only a small percentage of their total investment in the time it would take for an attack to be noticed and as a result they would likely lose out on future mining revenue if they were to attack the network