For every 1 ton of extra gas you carry, it costs around 30 to 40 kg to carry it. That is to say you will burn 30 to 40 more kg's for every extra ton. If the fuel cost differential is around 5% more expensive at the other end per flight sector hour, it pays to tanker gas around. Some one in the head shed works that stuff out.
They tanker to some ports but not many. It generally does not have anything to do with quick turnarounds.