Fable 5.1 is actually more expensive than 5.0 when run on the Artificial Analysis suite:

https://artificialanalysis.ai/#intelligence-efficiency-tabs

The tests measure Fable 5.1 (with fallbacks). The increased cost can come from Fable 5.1 triggering fallbacks less; which means less (cheaper) Opus when AA ran it.

IPO is nearing, must squeeze users more...