Broadly speaking, for an A320 every increase in 1000kg weight at optimum cruise altitude will result in 3-4% increased fuel consumption. It's similar for climb to the same altitude (obviously - the more weight to lift equals more fuel). The curious point is descent; heavier weights mean more fuel burn, but this tends to reflect greater aerodynamic efficiency at a given speed. At 300kts IAS a heavier jet will have a slower descent rate than a lighter one (closer to that sweet spot where the tangent hits the lift/drag curve) thus by taking longer to come down, the heavier jet uses more fuel but travels further in the process. Bit counterintuitive really.