Transaction Fee

A transaction fee is separate from the block subsidy: it's paid by whoever creates the transaction, to whichever miner successfully mines the block that includes it. Fees exist to prioritize transactions when a block has limited space, and keep the network incentivized to process transactions even as the block subsidy shrinks through future halvings.

For a transparent transaction, the fee is simply the difference between its inputs and outputs. A transaction touching the shielded pool can still have its fee determined the same way, from the public value-balance data alone, even though the shielded amounts themselves stay private.