Ex-post impact assessment of research investment on cassava production technologies was done to validate past funding on research based on economic surplus approach. Adoption survey on selected cassava production technologies conducted in the target states revealed that the average Adoption Quotient (AQ) was observed to be good in Kerala (56.34%), Tamil Nadu (63.63%) and Andhra Pradesh (51.20%). An economic surplus of Rs.3585.87 millions was generated in the target domain. Net Present Value (NPV) of economic gains was estimated to be Rs.3548.76 millions. Present value of research investment on cassava production technologies through cassava R & D system in the country were estimated to be Rs.37.11 millions. This investment resulted in the benefit cost ratio (BCR) of 96.63:1 with a high internal rate of return (IRR) of 104%. This research investment was proved to be highly economical and provided evidence to the policy makers that supporting further research investment on underground, under exploited tropical root crops like cassava is an economically viable proposition.