(Output / input), both of which are usually measured in money. If you can measure both of those things--and you have bigger problems if your finance department can't--it logically follows that you can measure productivity.

Measuring strictly in terms of money per unit time over a small enough timeframe is difficult because not all tasks directly result in immediately observed results.

There are tasks worked on at large enterprises that have 5+ year horizons, and those can't all immediately be tracked in terms of monetary gain that can be correlated with AI usage. We've barely even had AI as a daily tool used for development for a few years.