Good question - I never calculated the results (because this was not the reason for buying my Canon 1ds MK II).
Being lazy, I just took 2 images with 270 dl
pretty similar results, XL+XXL -> 8% of DL, 28% of earnings.
Just checked some more images, the percentage is correct only if an image sells reasonably well (for my images that means more than 70-80 DL).