Tankering
Very simple way to get it about 90% correct. Most aircraft burn about 3-5% of it's excess weight per hour. Lets call it 4%. So if the fuel is cheaper at departure airport it has to be at least 4% cheaper for a 1 hour flight for it to be economic. Likewise 8% for a 2 hour flight 12% for a 3 hour flight etc etc. This is a simplification but good enough for govenment work as the saying goes. You can understand now why pilots are discouraged from carrying extra fuel on long flights; on a 12 hour flight half will be burnt just carrying it. I operate Hawkers. If our crew always carried 1,000 lbs of excess fuel we would burn 40lbs /hr more. If we flew 500 hours per year that is 20,000 lbs, 11,330 ltrs @ 65p = £7,365. I know of some Hawker operators that have a policy of full wings always!