Cost simply isn't how it works, the mutations are completely random. It has no direction other than chance as to if it becomes airborne. People can talk about it but a strain this virulent going airborne would be a disaster.
I think that most of us understand more about ostrich than about virus. Let's make a think experiment. How difficult is that ostrich become airborne? How many mutation they need? Is it possible? Is it dangerous?
I hope there is an ornithologist nearby, but I'll try:
* Stronger chest muscles
* More white meat in the chest muscles
* Longer wings
* Better feathers
* Lighter bones
* Shorter and lighter legs, to reduce weight
* Less brain? (less weight)
* Do you need some special brain areas to fly?
Ostrich is a case of neoteny ( http://en.wikipedia.org/wiki/Neoteny ), so many features can be possible changed at one. But it's a old case, so the genes for the features needed to fly are probably partially overwritten with garbage since a long time.
A random walk on a gradient tends to go downhill, and becoming airborne is a long uphill trek.
Theoretically possible, but unlikely to the point I'm more concerned with other things.