Even when you ask $5 per month from a customer for maintenance and security (which WebARX pretty much automates for you), you'll get insane profit.
If you have 50 sites built and you provide $5 protection to your customers you'll end up making over $160 direct profit per month. Plus the reputation - customers will recommend you for caring for security. And you can do other type of up-sell when for example WebARX lets you know that site has SSL issues or something that needs improvements.
Most of the agencies usually ask around 10 dollars per month (or 100 per year) for maintenance/security package. We don't limit that. Depends how well you can sell your services, we have made selling security as easy as possible with the awareness articles in our blog which you can use to explain the importance of security to your customers.