It will eventually be very low, much lower than planes. The same technology that is being developed to make propellant on Mars can be used on Earth to extract CO2 from the air and/or sea water. A few squared km of solar panels on floating structures can produce enough propellant for one BFR trip a day.
The same propellant (methane) can also be used on planes, eventually, so BFR will bring many technologies advances independently of the outcome of the project.
The same propellant (methane) can also be used on planes, eventually, so BFR will bring many technologies advances independently of the outcome of the project.