I once did an exercise to determine the incremental fuel burn on DC10 operations to the Far East. I asked Flt Ops for the numbers, valid for typical weights. They took forever to come back, I remember that, and when they did I had a deputation of about five of them.
From memory, it was roughly 285 kgs per sector (flown via DXB) of fuel per additional 1,000 kgs of payload, i.e. 570 kgs right through to HKG. But this is 25 years ago...
Net result? Staff travel fares went up by 100%, from £30 round trip to £60.
But of course this is not definitive; as Fred implies, every flight will be different.
[BTW, this concept is far from academic. I'm aware of UK charter carriers lifting US-UK freight out of Florida at net/net revenue rates/kilo that won't cover marginal cost of incremental fuel burn].