Street voices
"You couldn't pay me to blank, so this is something that you'd never do, you'd never agree to do, even if someone pays you to do it, you're not doing that. So an example could be, oh my God, I'll pay you $20 if you stick your hand in the toilet, and then you'd reply, you couldn't pay me to do that, at all."
What it means
You say this when something is such a hard no that even money wouldn't tempt you. It's a dramatic way to refuse doing something gross, awkward, risky, or just deeply not your thing. The cash part is what gives it that extra bite, because the whole point is you're still saying no.
Usage examples
"They wanted me to jump in that freezing lake for twenty bucks. You couldn't pay me to do that, not even with snacks after."
"Eat oysters at a gas station for fifty bucks? You couldn't pay me to."
"They keep trying to get me on that sketchy fairground ride. You couldn't pay me to go near it."