People, cover artists do NOT make ten times what the author makes. If they make more than the author makes, then the author's book probably tanked really, really hard, and not even Mom bought a copy. The only place where this may get switched around routinely is in some magazines that pay relatively small amounts per word for articles and relatively solidly for cover art, and even then, I'd question it overall. It is definitely not true of novels, not even in the microcosm of small press.
The only possible place I can see this being true most of the time is in vanity publishing, where your artist is still expecting a coupla hundred bucks, and nobody's buying your book. Then, yes, the artist gets paid ten times what the author makes. But this is not normal, or ideal, or a good idea.
But don't take my word for it! I'm only at the very fringe of the industry, after all. Look it up! Writer's Market, Artist's Market, ask your writer friends, ask your artist friends, get the Ethical Pricing Guidelines. After all, if Jean Auel got a 100,000 dollar advance on Clan of the Cave Bear, maybe her cover artist DID get a cool mil! And if so, I want in.
But alas, that's just not how it works.
Thank you, that is all.