Patients going to a private hospital are charged the full amount, patients going to a government/municipal hospital are charged a subsidised rate and free services are provided to accident victims, unaccompanied unconscious individuals and victims of mass casualty incidents.
A hawk like focus on efficiency keeps the model financially viable. In 2007, the company had 10 ambulances in Mumbai[1]; now it has close to 100.
[1] http://www.acumenfund.org/investment/ziqitza-health-care-limited.html