Additive manufacturing has enabled new possibilities for realizing topology-optimized structures with complex geometries, while practical manufacturing constraints such as limited build volume still pose challenges for the fabrication of large-scale designs. Part decomposition has therefore attracted significant attention as an effective strategy for improving manufacturability by segmenting complex structures into smaller components that can be fabricated and assembled. Previous studies have investigated decomposition-based approaches in combination with topology optimization and demonstrated their potential for additively manufactured structures. Nevertheless, existing methods rely on heuristic redesign strategies or remain primarily focused on single-material configurations. This work considers a multi-material decomposition optimization methodology for topology-optimized structures under additive manufacturing constraints, with explicit consideration of additive manufacturing build volume and material cost. The proposed approach formulates the decomposition process within an optimization framework, in which part partitioning and material assignment are treated in a coupled manner. A build volume constraint is incorporated to account for manufacturability, while material cost is considered to reflect productivity, enabling manufacturing feasibility to be examined from both perspectives. Several numerical examples are presented to illustrate the characteristics of the proposed methodology and to discuss the influence of decomposition layout and material distribution under additive manufacturing constraints.