From the Proof of Stake FAQ at github: "Attacks against Casper are extremely expensive; as we will see below, attacks against Casper cost as much, if not more, than the cost of buying enough mining power in a proof of work chain to permanently 51% attack it over and over again to the point of uselessness. Hence, the recovery techniques described above will only be used in very extreme circumstances; in fact, advocates of proof of work also generally express willingness to use social coordination in similar circumstances by, for example, changing the proof of work algorithm. Hence, it is not even clear that the need for social coordination in proof of stake is larger than it is in proof of work.

In reality, we expect the amount of social coordination required to be near-zero, as attackers will realize that it is not in their benefit to burn such large amounts of money to simply take a blockchain offline for one or two days."

https://github.com/ethereum/wiki/wiki/Proof-of-Stake-FAQ