I am very excited to see NumPy and matplotlib coming along. I hope it comes soon!
Regarding pricing and in-app purchase, I have a suggestion: keep the price the same but offer the in-app purchase of a "warm glow". If you offered me the option to buy a "thumbs up" badge or a "like" for $5 I would totally buy a bunch of these when you roll out matplotlib. Apple's IAPs don't have to enable stuff; you can use them for stickers that show up when the program starts. I think many of your users would be happy to have the warm glow of knowing that they are supporting the development of an excellent product.