They were both privately held companies, so it might never have been disclosed.
Part of the acquisition was that NeXT would license Objective-C back to Stepstone for their own products, so it was more than an outright purchase anyway.
Tom Love mentioned it in a talk once (sorry, I’m not in a position to search for the reference right now). They were offered a choice between a fixed fee of some tens of thousands of dollars, or fifty cents per device. They picked the fixed fee, and felt it was the correct choice right up to the 2000s.
Interesting! Do you know what the "per device" was? Was that device with the Objective-C compiler installed? Device that shipped with the Objective-C runtime?
If it was device with the runtime, I imagine we'd have seen a rewritten "modern" runtime a few years earlier than we did.
I've found the video here https://www.youtube.com/watch?v=adI6-liGXqE, he said something along the lines of "five dollars [not fifty cents] per device running Objective-C", that make me think he meant the runtime.
I don't necessarily agree that the result would have been a rewritten runtime, just because the NeXT runtime (and compiler based on gcc) already was their own code so the licence must have been for "IP" in broader terms. Remembering that Apple were very heavily into Java around the time of the NeXT integration (https://www.sicpers.info/2020/07/so-whats-the-plan-part-1-wh...), I imagine that they would have gone more heavily in Java's direction.
Part of the acquisition was that NeXT would license Objective-C back to Stepstone for their own products, so it was more than an outright purchase anyway.