Not strictly unethical, depending on who you ask, but drawing porn. Someone told me their friend charged $300 for a single piece. Someone else told me they've known people who make $2-3k a month doing it full time. Or make a porn game that caters to a niche fetish. One dude whose game I played way back when went from taking donations via PayPal to having his own online store where he sells merch with his game's branding. The weirder the fetish, the higher you can get away with charging, though you'll also be limiting your potential client pool.