SpaceX indicated that will have orbital refueling in Elon Musks IAC 2017 presentation. Tanker versions of the BFS (top half of the BFR) will enable an orbital refueling business. Spacex may also buying propellants from in-space resources for refueling missions to Mars and other destinations.
An orbital propellant depot is a cache of propellant that is placed in orbit around Earth or another body to allow spacecraft or the transfer stage of the spacecraft to be fueled in space.
Ex-NASA administrator Mike Griffin commented at the 52nd AAS Annual Meeting in Houston, November 2005, that “at a conservatively low government price of $10,000/kg in LEO, 250 MT of fuel for two missions per year is worth $2.5 Billion, at government rates.” If one assumes that a 130 metric tonne launch vehicle could be flown twice a year for $2.5 billion, the price is about $10,000/kg.
Currently a Falcon 9 launch costs an average of $57 million, which works out to less than $2,500 per pound ($5500 per kg) to orbit.
As of April 2016, SpaceX had indicated that a Falcon Heavy could lift 63800 kg to LEO for a cost of $90 million a launch – which works out to $1410 per kg.
The target cost for the Spacex BFR is $5-10 million per reusable 150 ton launch. This would be $33 to $67 per kg.
SpaceX is targeting once a week launches and the new Falcon 9 block 5 could have 1-2 day relaunches.