Can you break it down in terms of the cost of opcodes?
(I'm sure this has been asked before. If someone can find it, please duplicate.)
From Ethereum's Design Rationale document:
21000 gas is charged for any transaction as a "base fee". This covers the cost of an elliptic curve operation to recover the sender address from the signature as well as the disk and bandwidth space of storing the transaction.
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. You can visit my blog to learn more about the concept of Ethereum Gas.