Is your argument that he ought to charge nothing because it's too hard to figure out what he's worth? If you're not willing to sort out what you're worth, you're dead in the water from the outset. Don't work for free. It's almost always a disastrous set-up. Your client doesn't have skin in the game. There's an emotional investment that comes with paying for something. It's a demonstration of VALUE. No payment, no value. It's pretty close to that simple.