Another way to do this would be to crowd fund. Instead of 1 person paying 50$ per book (just a random number, I dunno how much it costs) 10 people can pay 5$ each. 11th person onwards can get it for free.
You could also get some credits from these companies in return for advertising “this book is sponsored by blah company”
LibriVox does this with people who volunteer to read everything. It’s an extremely time consuming endeavor for the readers but they ultimately choose to do so unpaid. Why not create a system that covers the cost of the TTS model at cost as to avoid profiting off of what exists in the public domain? That’s the spirit of things being in the public domain.
You could also get some credits from these companies in return for advertising “this book is sponsored by blah company”