This guy https://extremehowto.com/install-a-solid-surface-shower-pan/ shows a solid surface shower pan going into a space straight onto the subfloor, and bare studs.
But I was talking to a design consultant with DIY experience who strongly suggested cement board or greenboard over the studs...
Since I documented all the items in my remodel, I figured I would share with anyone who may want it. Your prices may vary, as well as the quantities.
Hopefully it will help someone. When you unzip it, it is an excel spreadsheet.