The present study was an ex-post assessment done to validate past funding on cassava research, based on economic surplus approach. Though cassava was introduced as a food crop in Kerala, India from Brazil, it has changed its status to commercial crop at present in Tamil Nadu and Andhra Pradesh states. This was made possible due to intensive research and development efforts of different R&D organizations. Cassava technologies developed enabled in sustaining the crop in the country with the world’s highest productivity. All the cassava production technologies considered in the study resulted in an economic surplus of Rs.3585.87 million in the target domain covering Kerala, Tamil Nadu and Andhra Pradesh. Consumers and producers benefited to the tune of Rs.2090.64 million and Rs.1495.23 million respectively. Net present value of economic gains was estimated to be Rs.3548.76 million. Present value of research investment on cassava production technologies was Rs.37.11 million resulting in benefit cost ratio of 96.63:1 with a high internal rate of return of 104 per cent. Thus the study indicated that the research investments incurred on cassava production technologies development was highly economical and provided evidence to the policy makers that supporting the research investment on underground, under exploited tropical root crops like cassava is an economically viable proposition.