Word, yo!
People are willing to pay TK for versions of planes they can get for free here, but they're not willing to pay to help keep the free ones available??
Personally, I'll go for "packs" of planes, like say an F-14 pack which includes all 3 models or a MiG-23 pack with all its variants to a high level of detail, for no more than $10.
However, the blunt truth is that TK's planes are (as mentioned elsewhere) missing several crucial features of the real planes notably in the ground attack avionics and as such are limited in how good they can be. This is the engine's limitation, not that of any plane, and it needs to be fixed there.
I can't see going for an individual plane unless there was something extraordinary about it, and I can't imagine what that could be.
Unless these planes are priced under $5 each, then maybe, but I doubt he'll go that low. Right now, each new SF2 release is a new (updated) terrain plus a handful of planes (if you're merging them with previous releases all the planes included aren't new) for $20. That's a good model as terrains are the hardest to do and we have the smallest selection of those available, so you're basically paying for the terrain and some new planes that are appropriate to it. Take out the terrain and I think the value is cut in half. Lower the number of planes even more and the value drops too.