As I already wrote, at this moment a Bitcoin 51% attack probably costs 1,2-1,5 billion USD (or about 375.000 Bitcoins assuming an exchange rate of 4000/BTC, which are ~2% of the current Bitcoin supply).

In PoS, according to the theory, you can attack the network only with 51% of the supply - BUT that is assuming that all coins are used for "staking". That never occurs. In NXT, some months ago they calculated the staking amount at about 25 to 30% of the total supply.

So with 30% "staking" coins, you could attack the chain with almost 100% success rate if you buy 25 to 30% of all coins (and not 51%), but probably 15-20% would do the trick.

But now comes the Nothing at stake (N@S) problem. This problem enables a series of attacks with less than 51% of the staking coins. The best-known attacks are the so-called "bribing attack" and the "old keys attack" (or "history attack"), you can google them easily. Both involve some social engineering, but could probably achieve an attack with 5-10% of the supply. There are also short-term N@S attacks, that's why in PoS currencies more confirmations are needed to achieve a similar security level.