How much are the investment bankers earning on this?
I think nothing.
It's a direct sale, so the insiders (including the employees, not just the execs) get to sell however many shares they want directly to public shareholders.
So the company doesn't make any money either, just the shareholders. I suppose at some point the company itself could sell shares, and then maybe a banker would get involved, but since it would already be publicly listed, I'm not sure they'd have to be.