Well, seeing as now we shall finally be able to grace the skies of Azeroth with our flying mounts, do you think it would cost?
When you buy flying from whoever in Outland, you are just buying 'Flying', nothing more - but the ability to fly. Then you can buy artisan flying, and that is with the ideology of that you are becoming better at flying, and that you now have the ability to fly bigger, faster, and better mounts.
When you buy flying from whoever in Northrend you are then buying "cold weather flying", which suggests that you need to train yourself, and pass that onto your mount, the knowledge and skill of trying to manoeuvre your way though the cold of Northrend.
So, do you think it will cost - and then how much?