complexity

I did say I thought the odds were against it being what she intended, but I disagree with your estimate of the odds (which I suspect you would say are no chance at all).

Austen wasn't writing conspiracy stories or mysteries, so looking for complications in the text seems like a stretch, but she wasn't an unsubtle writer, either: she didn't provide a schematic for the characters. For instance, canon Miss Bingley is more subtle than most fanfic makes her (I've seen this done in some fanfic, and enjoyed it a lot), and I believe Miss Austen had a fuller picture of Caroline Bingley in mind than what she put on the page. In that same vein, I think it is quite possible (though somewhat unlikely) that Miss Austen was making fun of the way people take gossip and run with it in the matter of Darcy's income - if she wanted to nail something down, she could easily have put in a sentence of two when she was discussing the Bingley family fortune. I would go so far as to suggest - if she was playing with the idea of gossip in this instance - she might not even have a specific number in mind for the character: she might have thought he was "rich enough", and otherwise not care.