As of this writing, the average bitcoin transaction fee is over $30, according to this chart. However, I just bought $20 worth of bitcoin on coinbase, and they charged me a fee of $1.49.
My understanding of the fee for a transaction is that it is a function of market forces, the number of bytes in the transaction, and the time you are willing to wait for your transaction to be added to the blockchain. Overall however, I had the impression that at a given time these fees are relatively constant per transaction. So I would expect that coinbase would have to spend something around $30 in fees to give me my $18.51.
What don't I understand?