I remember a similar discussion with a manager in a company that paid bonuses at his level.
Me: Whats this bonus scheme?
Him: If we hit our targets, a bonus gets paid to some members of staff
Me: Those members being yourself?
Me: And what do I get for working harder?
Him: The satisfaction of a job well done.
Yea, because I'm in here for the love of the job and the free biscuits in client meetings. I'd do it for free if you let me.