Countries considering participating in a REDD+ mechanism need information on what it would cost them to reduce emissions from deforestation and forest degradation. This study was conducted to estimate the cost of managing forest carbon under REDD+ initiatives in Kolo Hills Forest, Kondoa, Tanzania. Socioeconomic and biophysical information was collected through structured questionnaires, focus group discussions, and forest inventory, respectively. Results show that the community participated in managing the forest by undertaking a range of activities such as tree planting, patrolling, and fire protection. The estimated total cost was USD 418,349.38 while the average cost was USD 79.06/ha. The average carbon stored was 19.75 tC ha−1, which is equivalent to 72.48 tCO2 ha−1. Costs incurred by managing the forest in relation to tCO2 stored were USD 1.0485 tCO2 e−1ha−1. The project was found to be economically feasible at 5%, 10%, 15%, and 20% discount rates with NPVs of USD 107,102,331.83, USD 33,986,255.86, USD 10,312,945, and USD 1,245,905.11, respectively. The internal rate of return was 21.21% which is much higher than the World Bank rate of 15.8% and the Tanzania rate of 14.8%. We therefore conclude that the decision to undertake this REDD+ project was worthwhile and should be favoured against the “do nothing” alternative.