Renowned visual artist Donato Giancola has published a series of posts on his website Donato Arts, in which he criticizes Wizards of the Coast in the harshest terms. "You should be ashamed Wizards of the Coast," he wrote in the first of three (to date) open letters to Magic: The Gathering's publisher, which he describes Wizards of the Coast, Hasbro, and Disney's Marvel as defining examples of "corporate exploitative practices."
So it seems clear that WOTC is spending less than 1M on art per set. That is less than I expected.