Even the highest page rate for Big 2 writers (if memory serves me right) is about $120/page. So Snyder would be making ~$2.5k/mo from Batman singles (not sure how TPB royalties, any sort of comps or sponsorships, etc work), before taxes. Not too shabby all things considered. (http://www.makingcomics.com/2014/10/...tists-writers/ for some confirmation. Google probably has more results)
From my understanding in the past, Image used to take a flat fee up front for publication. Per their site, now they take a "small/modest fee" each issue. Per Jim Zubkavich (http://www.jimzub.com/the-reality-of...-owned-comics/), Image takes $0.80/issue. But overall, he says $0.30/issue is made after the publisher, Diamond, and retailers take their cut. Not sure if he's using cement numbers or just using some broad examples.
Either way, $0.30 x 15,000 (around the Low, Descender, Klaus sales range...solid indie sales) comes to $4.5k. That's split between writer, artist, colorist, letter, etc, however they agree upon. I'll presume colorists and letterers don't demand much, they do bulk work anyway. Also, lots of indie artists color their own stuff and writers even letter their own work. I'll presume a writer makes in the range of $2k-2.5k from a 15k/mo indie seller. If you're Hickman, BKV, Kirkman, etc, you're selling in the 20k-60k range, and probably selling another title or 2 as back up. So that's much more $$
So...
If you're an ELITE Big 2 headliner writer, you'll make ~$2.5k/mo per title
If you're a SOLID indie writer, you'll make ~$2.5k/mo off one title
If you're an ELITE indie writer, you can be making $7k~10k/mo off 2-3 titles
Of course, these are not definitive #'s, but ultimately, indie writing isn't guaranteed more money. But the ceiling is much much higher.