None of us can determine which aircraft is more economical because we are not provided with enough information. Your logic seems reasonable but there are other costs, such as maintenance, dispatch reliability, initial procurement costs, extra crew costs, just to give a few examples.
Based simply on fuel vs load, the A380 might be more economical provided the load factors are high. I suspect a B777 is more cost effective at 80% load factor than an A380 at 80% load factor. Isn't fuel about 30% of an airlines cost, with employee costs running a close second? The cost of fuel varies wildly too, such as in the past year.
My whole point, is that only a few of the bean-counters and the bosses really know which aircraft types are performing the best. I would bet though, the B777 surpasses any Airbus type when it comes to economics.
As an aside, can anyone give a reasonable explanation why statistics show that Airbus pilots generally take more 'additional' fuel than their Boeing counterparts? It might be something as simple as the differences in route structure, but I have often wondered about the discrepancy.