What's being staked is ETH (or any other units of PoS-based cryptocurrency) itself. The lock-in, and therefore any opportunity cost associated with the locked-i

What's being staked is ETH (or any other units of PoS-based cryptocurrency) itself. The lock-in, and therefore any opportunity cost associated with the locked-in value, is the price to play. The analogy, to my understanding, is not 100% faithful to how PoS works, at least in Ethereum. There's no choosing one of the bricks, or one of the units of currency or anything. You simply lock in your units as collateral for your right to take part in the dice roll selection where, if chosen, you get to add a new block to the blockchain. If you append a block without cheating, you are rewarded with new units of the currency. If you cheat, your collateral is taken away. You may choose to keep your stake in as long as you want.