What is the "0x Protocol Fee"?
Beginning with the launch of 0x v3, all transactions using the 0x Protocol will include a protocol fee. This fee is paid by the order taker—the user who sends a transaction to fill a 0x order—at the time the transaction is executed. Fill transactions that include an insufficient fee will fail.
The protocol fee is calculated based on the price of gas used in the transaction. It is always paid in ETH and is included with the gas fee when a transaction is sent to the blockchain. As with gas, if more ETH is allocated to a transaction than is necessary the remaining ETH is sent back to your wallet when the transaction is mined.
What do I have to do to pay the 0x Protocol Fee?
RADAR RELAY will automatically include the protocol fee in your transaction when you execute a trade.
Because the protocol fee depends on the gas price you select, transactions on RADAR RELAY allocate a little extra ETH to cover the protocol fee so that the transaction will still succeed if you increase the gas price—don't worry, any extra amount is returned. You can use a gas price value twice as large as the *high* gas price quoted on https://ethgasstation.info/; any higher than that and the transaction may fail.
Why was the Protocol Fee added?
The protocol fee was added in an effort to make development of the 0x Protocol self-sustaining. The more useful the protocol, the more orders are filled, and the more fees accumulate to build new user and developer tools, support market makers, and add new features.
The protocol fee is currently added to a pool that is distributed back to market makers—users that create and post 0x orders—using a mechanism that requires market makers to manage a staked pool of ZRX. Anyone who holds ZRX can participate in this staking process and potentially receive some of the distributed fees.
For more details on the staking process, see https://0x.org/zrx/staking.