Air freight ain't cheap. But a volume that size would be done by contract and negotiate at a price only available to those that ship at such volume.
And the price would still have to cover stuff like jet fuel, which (as you should know) make up the majority of the costs. And yes, these boxes are huge and heavy for air freight, not to mention having very low to margins.
Your contract price may come at 25% off or even 50% off, I don't know, but it's not going to be cheap and margins are already as low as they are.
Giving a (relatively) low estimate of 15 USD air freight per box and, lets say 500k units over that month, you're asking Microsoft to pay up 7.5 Million USD just so they can move up the supply line by 2 weeks.
Guess what? 2 weeks later you won't have those boxes coming in by ship because they have been air lifted! So you're still going to have a massive headache 2 weeks later.
That's 7.5 Million dollars down the drain to solve an issue and at the same time creating another problem because you won't have the boxes coming in by ship.
In the end they won't have solved the short on supply issue by delaying the backlash by 2 weeks, if there is any backlash. If they have too little units 1 month into the cycle, airlifting 1 month's worth of supply will do nothing to help it.
Plus MS has a Xbox manufacturing base in brazil and Xbox OG/360 were manufactured in Mexico and Hungary as well. There is nothing that says that MS is solely dependent on China for production especially since production in china isn't as cheap as it used to be.
Manufacturing in Brazil is to fix the Brazil import issue, and is done in low numbers.
I seriously doubt they will assemble them anywhere but China at this phase of the console's cycle. Especially when it's Foxconn doing the assembly.
Furthermore unless you know the intricacies of MS shipping contracts you can't make a definitive statement on cost. A shipper may have offered to float air freight cost for late launch shipments to go by air in an effort to win the bulk of the shipments over the life of Xbox one.
Airlifting consoles, if they really do it, only are done to limit supply issues at the launch. There won't be any more airlifting once stock starts to be readily available, so you're not really going to get any chance to win any long term air freight contract.