The Sistina is a great illustration of what I'm talking about, actually.
Michelangelo spent something like 12 years on and off planning, painting and adorning the Chapel.
He made thousands of sketches and studies and went far above and beyond the original terms of his contract. No one can honestly say the commission he received for the Sistina was adequate to the time and effort he invested in it, let alone made it a profitable transaction for him.
If I'm not mistaken, he was paid more for the Paulina nearby, and it's never even metioned among his great works - most people don't even realize he was the painter, and compared to the Sistina it looks very uninspired and generic.