My understanding is that sameday ACH does not cost extra (from a network perspective) than regular ACH, it's just that some shitty banks apparently like to charge extra for it.
Outside the AI summaries, I couldn’t easily find the sources. But a couple of different sources say same-day ACH is 5.2 cents per outgoing item while overnight is 0.35 cents.
What I can say is the Fed has draconian rules for pricing its services. The services cannot be subsidized but must be priced to recover costs, including a return on capital similar to what a private payment system (ie from the Clearinghouse) would expect.
This seems to be the official pricing info: https://www.frbservices.org/resources/service-fees/ach-2026/
I read it as the sending institution pays 5.2 cents to the receiving institution per item, on top of the 0.35c/item + 0.1c/item same-day surcharge for a maximum of 5.65 cents/item before volume discounts.