How many copies of The Prince were printed in Machiavelli's lifetime? What did he make in royalties from his most famous and enduring achievement?
Answer: nothing.
Isaac Asimov one of the world's great writers? There's not much consensus there.
Citizen Kane is the essence of Welles' legacy. He knew it would be a complete flop - he actually misled RKO executives about the plot, budget, etc. just so they would make it - and it was a complete fail ure at the box office.
Dali and Picasso are excellent examples of artists who got paid well for their substandard work at the end of their lives while getting paid nothing for their truly great work in their prime.
As I pointed out above, Twain made his money as a journalist - his classic writings were done for his enjoyment at his own pace.
Mamet is a prime example - he himself says that he wrote or cowrote his less inspired scripts (like the Costner vehicle Untouchables) for a paycheck so he could finance his true legacy: Glengarry Glen Ross, etc. And even in movies like that he acknowledges that not only do they lose money, they would have lost more if they didn't have stars like Pacino, Lemmon, Steve Martin, etc. in them.
What Mamet considers his greatest work he produces knowing full well that he will lose money.
You then proceeded to do just that. Every one of the people I mentioned lived or do live on the proceeds of their art. They profited.
The relative greatness is a matter of opinion. Glengarry Glenross sucked. Joyce's writings are 12 years of wasted drivel, while Asimov is a seminal figure of an entire genre of writing.
See, opinion is useless to support an argument. Merely by stating that any given example is not "great art" does not make your opinion valid.