Operating expenses to run PoS networks are wayyyyyyy lower than PoW. The cost of running software will continue to approach 0.
As I discussed in Endgame: Proof of Governance , and many others have before me, the superpower of PoS security is really accountability. If you double sign, everyone knows who you are. We tend to even know their real-world
For example, if you hold Ether you can stake it to help secure the network once Proof of Stake launches. In return for Staking your ETH, you get paid in more ETH, at a rate of about 5%.