Any transaction has a base fee of 21,000 gas in order to cover the cost of an elliptic curve operation that recovers the sender address from the signature, as well as the disk space of storing the transaction, according to the Ethereum White Paper.
https://ethereum-magicians.org/t/some-medium-term-dust-cleanup-ideas/6287
Why do txs cost 21000 gas?
To understand how special-purpose cheap withdrawals could be done, it helps first to understand what goes into the 21000 gas in a tx. The cost of processing a tx includes:
protocol_params.go#L31
The minimum transaction value of 21,000 gas may change in the future, so it is recommended to make this value updatable.
Add this code;
