How much money would a modern-day playwriight or author make?

Answer 1

Just like with any creative avenue, it depends on many factors, particularly whether or not they are any good at writing.

Who you are writing for, and how you explore that avenue will also have an impact- who is your audience, and how are you going to reach them? Many theatres nowadays have new work festivals to develop and promote new plays. Some playwrights go the route of fringe festivals and one act festivals. It is possible to make a decent living as a playwright, to be very successful even. But it is definitely a difficult start.

Answer 2

From zero to millions depending on a host of factors.