Fable is twice the size of Opus from what I gathered. So I'm not sure if 2x price translates to 2x profits as well.

Not sure about GPT but it seems plausible they've also been increasing the model size with recent releases. (Progressively training a bigger model and easing into a profitable price range for that model scale?)

it looks like all AI is following the same pattern as GPT-3 again, building bigger models to achieve better results.