Seriously, manga-ka (ALSO KNOWN AS COMIC BOOK ARTISTS) get paid shit. Ever notice all the ever bitch about in their omake is how little sleep they get, what a shitty place they live in, and how stressed they are? They're not lying. No comic artist gets paid much unless they're REALLY something new, and this day and age that's not easy.
And remember, they get paid for SELLING their work. The characters, the setting, everything to a corporation that can do whatever the fuck they want to them.