I'd guess it's to do with the fact that with the spitfire, you're just buying another spitfire, of which you already have half a dozen unlockable for free. With the Ascender, it's a completely new and unique plane that looks cool. Expect a bit of premium for it. Although don't ask me why the XP-50 isn't more expensive.
EDIT: Well, the Pe-3 is as tough as ever. I do a dive through a furball at around 550 km/h aiming for a ground target but taking pot shots at anything flying near my nose. I get a kill and a couple of non-critical hits this way. I finish my run, dropping my bombs but doing no real damage loop up and around for another pass. This time everyone is paying attention to me, I hear hits striking the plane all over but I'm surprised as every time I glace over to check my damage, the plane silhouette hasn't even appeared yet. NO DAMAGE. I set another plane on fire in a near head on pass and my gunner gets a few hits on another. I'm just dogfighting now, waiting for my bombs to come back up and something I hit finally crashes getting me another kill. It's around this point where something, probably with a cannon comes in and starts doing damage. It's nothing unsurvivable for the moment, but I'm low, I've got my bombs back and I've still got a ground target to hit, so I make another run for a couple of medium tanks and release my bombs low. I'm damaged enough and slow enough at this point, I can't get out of the blast. I ALMOST take out some planes with me, but the tanks survive and I crash.
tl;dr: Pe-3 says, "You'll shoot me down when I SAY you can shoot me down."