
The Quote Investigator http://quoteinvestigator.com/tag/virginia-woolf/ traces the origin of such statements to The Intimate Notebooks of George Jean Nathan (1932), where the diarist states:
We were sitting one morning two Summers ago, Ferenc Molnár, Dr. Rudolf Kommer and I, in the little garden of a coffee-house in the Austrian Tyrol. “Your writing?” we asked him. “How do you regard it?” Languidly he readjusted the inevitable monocle to his eye. “Like a whore,” he blandly ventured. “First, I did it for my own pleasure. Then I did it for the pleasure of my friends. And now — I do it for money.”
Misattributed