Emerging Internet mega-constellations promise to provide broadband network service globally. Typically these constellations follow a uniform structure and functionality model, in which the constellation is evenly organized at scale, and each satellite equally serves for both access and forwarding tasks. However, due to the inherently non-uniform distribution of terrestrial user demands, the uniform constellation design not only suffers from low network utilization but also involves significant operational costs for constellation operators. This paper studies a forward-looking yet important problem for future Internet mega-constellations: how can we accomplish cost-effective constellation design, i.e., satisfying ubiquitous user demands while improving constellation-wide network utilization and optimizing operational cost? To this end, we propose DIV, an internetworking mechanism, which attains the cost-effective goal by dynamically adjusting the constellation-wide inter-satellite connectivity and per-satellite functionality based on spatial and temporal varying user demands. Specifically, DIV divides satellites into backbone and access according to the inherent difference between Internet local and interregional traffic patterns, and reorganizes the constellation structure to optimize network utilization on-demand. Extensive simulations based on realistic constellation information demonstrate that DIV can achieve cost-effectiveness for various state-of-the-art mega-constellations and increase the network efficiency by up to 2.5 × while guaranteeing stable, pervasive network services for geo-distributed terrestrial users.