Why is PoS so expensive?

Could you elaborate on why the implementation of Proof of Stake (PoS) in cryptocurrency networks tends to be costly? Many have observed that the transition from Proof of Work (PoW) to PoS, while aiming for greater efficiency and sustainability, has often come with significant upfront investments. Are these expenses primarily due to the technological complexities involved in setting up and maintaining a PoS system? Or are they driven by the high value placed on staked coins, which serve as the backbone of the consensus mechanism? Additionally, does the cost vary significantly between different PoS-based blockchains, and if so, what factors determine these variations? Clarifying these aspects would help us better understand the financial implications of adopting PoS as a consensus protocol.