I have zero inside knowledge on this; no one discussed with me the ideal price point for Rebels (and I didn’t expect them to) or the criteria CoG use to make that call. But looking at the examples you guys are discussing – Slammed vs Hero Unmasked, for example, or the variation in the HR books – I’d make a wild guess that CoG is setting prices according to demand. The games that are most popular get a price bump. That’s also consistent with Choice of Robots costing $9.99 or whatever it costs these days.
Moral of the story? Buy your copy of Rebels now. 
it’s interesting to read over this old thread again, especially Jason’s explanation of CoG’s thinking way back at their second ever face-to-face company meeting. It reminds me how much inflation there’s been not just in prices but in expectations of game length (“Similarly, when you start to get into the neighborhood of 100k words, we think that’s worth $3… Heroes Rise is a very long 110k words,” and yes, I remember just how long it felt to me when I first read it one hot night in Ed Daein).
Jason links to another forum member’s analysis of CoG’s payment terms, which is worth reading in full. It’s basically all still valid, and ends with, “What this is all leading up to, is to point out that one of the main reasons CoG needs to charge for games is specifically so they can pay their writers well. It isn’t there so Jason can drive limousines into swimming pools and do Scrooge McDuck style dives into large piles of cash.”
Anyway, Jason’s posts on that old thread testify to how much thought CoG puts into the pricing of their games. While they’re more close-mouthed about it these days than they were back when they first incorporated, I’m sure they’re looking at things like the impact on sales rates and piracy.