but to do accurate billing, they would have to tell us when our usage triggers a guard rail and causes our query to be rerouted or denied.

And if they do that accurately, there's no stopping someone from mapping the guard rails.

Clearly that outcome is more uncomfortable than what we have now, which is enough people seem to be okay with vague billing.

How would that work? It's not simple to set a price, when you have a fixed asset. Seems more complicated to set price on a product (ChatGPT), where the product is just a shell for something else entirely (the model), that changes by the month.