Putting a cost on the carriage of "excess" or discretionary fuel is difficult for several reasons. It's tail-chasing exercise.
1. How much fuel is burned to carry that additional fuel? The theory does not consider the variables of an actual flight.
2. Less fuel may be required to be uplifted at the destination airport which has cheaper fuel than that uplifted at the point of origin.
3. Revenue payload can suffer. For example, 2000kgs of additional fuel could require the offload of a pallet of freight. Apart from the direct cost, what is the longer-term cost of this?
Unless additional fuel uplifts are considered habitually excessive, surely it's better to allow sleeping dogs sleep. I'm with Keg.