Desert,
fuel may not be 'subsidised' but the cost can vary enormously. We at easy have a complicated formula that accounts for cost of fuel at departure/ cost at destination/ increase engine use due to increased weights/ tire wear etc etc.
The amount of tankering does vary and I assure you the cost of fuel varies enormously at some airfields. Mostly it is due to the transportation costs of getting the fuel to the airport itself, which is passed onto the customer.
In such cases we will tanker to MTOW or MLW (actually minus 500kgs to allow for LMC's) but I've never had cause to block off seats to allow extra fuel carriage! Mind you, at .01p per seat it is prob more cost effective to do just that!!!