Michael-Angelo (one of the greatest artists of all time) is as good an example as any. But I think that he was commissioned to do the Sistine Chapel, he didn't paint the Chapel and then charge admission to it (I think the church took care of that).

If the book is to be believed then he was a sculpter at heart and didn't actually like painting much. He was indeed commissioned to do the chapel but I unfortunately can't remember whether he did it out of free will or whether he was pressed to do it.

Thanks for the book recommendation, thats my favourite way to find new material to read. I've put your recommendation on my ever increasing list of books to read this summer.