LOL I wouldn't even install Google Analytics on a site for $20. You are helping him get traffic and earn money.
He's not investing in you, he's paying you all he can afford. A good SEO client realizes that SEO is an investment. Maybe they can't afford in and stay in the black now, but that's where you come in to help them earn money.
I had a client who was in the red for 3 months and I sold him SEO. Of course the monthly expense put him even more in the red initially, but it was my job to improve his sales through SEO and bring up his profits. That's the sole foundation of an investment.
My hourly rate is $35. So $20 per month gets him roughly 34 minutes and 17 seconds of my time every month. Maybe to login to his Google Analytics once a week.