SLA: Service Letter Agreement for a small home based business

Recently I was approached by one of my clients that I occasioanlly do business with. Whenever this customer has a tech question or problem he calls me (like the rest of my customers) and then I go and service him. But now he is asking me for an SLA to do regular (maybe monthly) maintenance on his systems. How much do you charge a client on a monthly basis to do his systems maintenance. What do you include in the SLA? Should I start selling this SLA agreement to the rest of my customers? Or wait untill they ask for it? any ideas or suggestions are highly appreciated.

Thanks
Haas