Interesting concept for sure. Good on you for thinking through the interface and laying it all out in a workable framework.
Acknowledging the older demographic of many (most?) BiaB producers on this forum, many are hobbyists who do this as a retirement hobby, just for fun. I'm in this group, though I didn't start out there.

If a songwriter wanted to collaborate with me on a production, I'd do it gratis so long as it didn't require an extraordinary amount of effort. So if I was interested in your concept, I'd likely post a price of $0. Or maybe $1. But monetizing my BiaB fun isn't a priority. Having fun with whatever I do is.
I've not used AI to produce a song, but the results I've heard are often impressive, and in many cases comparable or superior to many BiaB productions. Suspect whatever gap exists will continue to rapidly shrink, and the pricing does not appear to be a barrier.
I'll be watching this thread with much interest.