i just did a simple calculation, about 35k bitcoins are traded on GDAX every day. At its current price of $6900 and a trading fee of 0.3%, the exchange puts about $700,000 in its pocket EACH DAY and it is ONLY for bitcoin. After including all other cryptos, this number can go up to more than $2MM. Since the exchange is active 365 days a year, the exchange's annual revenue can reach almost $1bn!!
This your calculation makes its kind of so straight forward but in the real sense, its not as straight forward what you consider is only the revenue which might not even be up to that before the need to start factoring cost which can eat into that revenue really fast. Before the payment of staff who have to be on rotation to work 24hours due to the nature of the clients they service all around the world, the amount they have to incur to ensure that the site available at every time, what of the cost of security, wallet maintenance, hosting, licences, legal fees based on retainership, heavy equipments, database management and other ancillary costs. Its more than what we imagine.