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Assume an intern would get $10 an hour or $400 a week. If you think a typical competent intern would generate $2,000 a week in revenue, then you'd want to set commission at 20%, or ideally 25% to compensate the intern for the risk they're taking.

The thing is, though, that your losing your time and focus is a big opportunity cost of hiring an intern, so you shouldn't hire someone you doubt can do what you want. Hire carefully, pay a normal salary and you'll be happier and also attract better candidates.