With the rapid growth of aquaculture, some negative factors of extensive monoculture accumulated, causing integrated multi-trophic aquaculture (IMTA) to draw increasing attention worldwide for its ecological and economic advantages. However, the development of IMTA in open water systems may not go as smoothly as anticipated—at least in Weihai, China, some producers who have adopted it would prefer to return to monoculture. This paper explores the problem from the angle of the economic performance by providing an in-depth analysis of costs and revenues generated by kelp monoculture and kelp-mollusk polyculture (IMTA). We find that the monoculture generates higher profits during the same production cycle. Besides inherent defects in system design, the rapidly growing labor cost and depressing selling price of the mollusk conspire to aggravate the economic failure of IMTA. Due to positive environmental externalities, the social benefits associated with IMTA are higher than the private benefits. This implies that there is a role for the government to generalize IMTA to achieve more total benefits. But single policy such as subsidies may backfire, a combination of policies designed to promote IMTA could be effective.