Bitcoin miners compete to earn a block reward by applying hashrate to solve a puzzle set by the code. Equilibrium hashrate is a function of, inter alia, hashrate cost and the value of the block reward. The marginal effect of a decline in the block reward reduces hashrate. A reduction in hashrate reduces the cost to attack the network. The block reward is the sum of newly minted tokens (the "coinbase") and fees offered by transactors. Historically and presently, the majority of the block reward has come from the coinbase. This cannot continue indefinitely, however, since the coinbase halves approximately every four years. We estimate a structural model of transaction fees that facilitates causal understanding of the fees that have been observed and insight into the direction of fees as the coinbase declines in significance.