I would price based on category (short, novella, novel) rather than length OF the novel.
Also, let me throw this out there:
I rarely buy short stories as stand alone, but if I do, NEVER for more than the .99, which I assume is minimum other than free.
I will pay up to around 2.99 for a novella, but I usually only buy them from authors I like or from publishers.
I DO NOT generally buy ebooks priced at 2.99 and below because that screams "I self-published this and I'm not sure it's worth Real Money" to me. My favored price range to see for ebooks that are not from the Big 6 is around 4.99, give or take. To me, that $5 says "Yeah, I know I don't have Tor backing me up, but I'm confident that this is worth it."