That's a weight saving per aircraft for RR of 2 metric tons (5.5 tons wet?); that's a lot of fuel/payload. GE must have offered the juiciest deal imaginable to sell that to Emirates; two extra tons (about 1% [2.5% wet?] of the aircraft's maximum fuel load) hauled all over the world throughout the service life of an aircraft is a large amount of cash.
Interesting, but you left out an important part - what's the relative fuel burn between the two engines? Even a 1% fuel burn advantage is huge money over the life of an airplane (especially a long-haul like the A380). Unfortunately both the engine companies and the airframe manufactures consider fuel burn numbers to be proprietary, so unless you're a legitimate potential buyer they tend to be pretty tight lipped. So while I have knowledge about relative fuel burn numbers on some Boeing models, I can't repeat them - and I obviously know little about brand A fuel burn numbers.