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.