This study argues that an effective response to the Carbon Border Adjustment Mechanism (CBAM) requires integrating external carbon costs into firms’ internal investment appraisal and resource allocation systems. To this end, it proposes a management accounting-based internalization model for industries subject to CBAM. The model consists of three stages: pricing, evaluation, and allocation. In the pricing stage, an appropriate internal carbon price is estimated by considering both the economic viability of low-carbon investments and firms’ regulatory exposure to CBAM. In the evaluation stage, this price is incorporated into carbon-adjusted investment appraisal to assess the long-term value of low-carbon investments. In the allocation stage, an internal carbon fund links the cost burden of high-carbon business units with financial support for low-carbon transition activities. Using publicly available data from a case firm and literature-based assumptions, the study conducts a quasi-real simulation. The results show that substantial CBAM-related costs may arise even with limited EU market exposure when the gap between EU and domestic carbon prices is large. This suggests that internal carbon pricing should be used not merely as a disclosure item or symbolic policy, but as a practical tool for investment appraisal and internal resource allocation. This study contributes by providing a decision-making framework that helps CBAM-exposed firms balance the short-term costs and long-term benefits of low-carbon transition. It also reframes CBAM response as an industry-level policy issue requiring infrastructure for carbon cost internalization, rather than merely an individual firm’s cost burden.