Actually if you compare fuel burn and maintenance costs the A340 is cheaper to operate than all models of the 777 aside from the base 200 model. Cheaper to purchase also. Your point about the 777x being in the same fleet group is one i hadn't thought of and should be considered.
I think the fuel consuption displayed can be misleading. I had a few surprises with them. Reality did not match my excel sheets. IIRC, Sami once said that what was shown was an average, for the nominal range of the plane. As a plane that flies shorter can take off lighter, it will drink less. So, if a plane with 2000NM & a plane with 4000NM have similar displayed fuel consuption, the second one, in fact, drinks less - for flying the same route. It matches my experience(The A148 is a good plane, but flying the A148E above 1800NM makes the fuel drinking kill you). So, the 777 does not drink as much as is seems, compared to shorter-legged planes. Unfortunately, I don't know the formula to normalize for range flown. I know my excel sheet is wrong, but I don't know how to correct it.
For the purchasing price, it depends on the game. In last GW3, the MRJ costed 30M$, in the previous one 65M$. Demand has a huge impact, which makes that kind of question very dynamic. But yes, the 777 being a Rolls-Royce, it is always in great demand, and therefore very expensive to buy/lease.
Last point : when I'll see the in-game characteristics of the 330 neo that has just been implemented, I might consider it as a full replacement like the 777X... or not. We'll see.