How does ISRO keep the cost of its space missions low?

According to P Sudarshan, who worked in ISRO from 1976-1988 and was responsible for budgeting, the space agency operated on zero-based budgeting along with other means. Zero-based budgeting is a method where the previous year's budget isn't taken as a base for the current and future years. The budget estimate is stated after starting to calculate from zero.