Writing is like prostitution. First you do it for the love of it, then you do it for a few friends, and finally you do it for money.

Ferenc Molnár (1878-1952) Hungarian-American author, stage director, dramatist [a.k.a. Franz Molnar]

Quoted in George Jean Nathan, Intimate Notebooks (1932)
    (Source)

Common form of a quote often misattributed to Molière. It original version actually appears to have originated with Molnar, who, when asked how he regarded his writing, answered, "Like a whore. 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."

More discussion here.