How much does it cost to reconfigure the tanker to a charter passenger configuration then convert it back to tanker configuration?
If the body fuel tanks are all below the cabin floor, none.
When the A-330 MRTT was selected for the KC-X program, one of the KC-X requirements was the ability to be reconfigured to passenger, cargo, or medevac missions in no more than two man-hours with no more than two personnel.
The KC-45A met that requirement. The passenger configuration is palletized, and rolls into the cargo area section by section and locked into the cargo floor.
However, I do beleive the RAF Voyager is not even equipped with a main deck cargo hatch, and remains in a passenger configuration at all times.