Current market cap of ETH is ~$324B, thus getting 50.1% of ETH would require $162.3B in capital. However, as soon as you start acquiring ETH the price will increase, especially at those large volumes.
It would be insanely hard to come up with enough resources to buy enough ETH in a POS world to take over the network. Never mind the fact that as soon as it's become evident you've taken over the network the value of the network is essentially worthless and you've just destroyed billions of dollars worth of capital in the process.