I think there's two sides to it: 1) how difficult is it from a monetary perspective and 2) how difficult it is from a practical/implementation perspective.
Whilst I agree that it's difficult to acquire and manage the resources to attack the network (and therefore much more difficult form a practical/implementation perspective), I'm not so sure that the combined cost (monetary value) would exceed 51% of the supply of coins (which is what it would take under PoS).
It depends. Are there already people willing to give others control of their coins, the answer is yes. Major exchanges tend to hold disproportionally larger amount of coins as compared to the normal holders because people almost always store it to stake it with them.
I really don't think the monetary perspective is a huge factor, or rather we're looking at it from the wrong point of view. Instead of thinking which is more expensive, either in monetary terms or execution, perhaps we should see which gives sufficient security such that attackers are not incentivised to attack either. If it is infeasible for both kinds of implementation, then it's great. There isn't a need to compare which is more secure, because they are fundamentally different.