The sustainable utilization of black soldier fly (BSF) for recycling organic waste into high-quality protein feed and organic fertilizer with a low environmental footprint is gaining momentum worldwide. Although BSF farming is becoming a rapidly growing agribusiness, studies on BSF farming's economic aspects are limited. This study analyses the economic implications of farming BSF for animal feeds and frass fertilizer (FF) production to provide evidence of the economic benefits using experimental data. The BSF larvae were reared using brewery spent grain amended with sawdust, biochar, and gypsum, to determine the best waste combination for cost-effective production. The agronomic performance of FF on the maize crop was assessed using field experiments. Our results demonstrated that the rearing substrate accounts for 81 - 90% of the total BSF production cost. The utilization of frass fertilizer as an additional value-added product would increase farmer's net income by 5 - 15 folds compared to BSF farming alone. Feedstock amended with 20% biochar increased net income by 10 - 64% for BSF larvae and FF production compared to other feedstocks. Production of 1 tonne of dried BSF larvae (US$ 900) would generate 10 - 34 tonnes of frass fertilizer worth US$ 3000 - 10,200. Maize grown on plots treated with FF yielded 29 - 44% higher net income than plots amended with commercial organic fertilizer. Furthermore, smallholder insect farmers' direct use of FF for maize production would generate 30 - 232% higher net income than farmers purchasing similar FF. Our results demonstrate for the first time the role of insect farming in circular economy and justify the opportunities for future investments that would lead to enhanced sustainability for agricultural and food systems, especially for smallholder farmers in low- and middle-income countries.