There are plenty of rewards that aren't monetary. The author isn't saying you should work in a job that provides no benefits. He's saying the benefit of money shouldn't be the primary one because other benefits (like fulfillment) are more important.

So the point of the mental exercise is to help you find a job that is worthwhile regardless of the money you're paid to do it. So you'd still be rewarded for your efforts just not in money (which again wouldn't mean anything to you anyway if you had a billion dollars in the bank)