This paper examines the choice between direct and absorption costing in a cost-based transfer pricing system for duopolistic firms competing with product market prices. Existing literature has shown that the adoption of an absorption costing system, which drives up the intrafirm transfer price, strategically dominates direct costing for the two firms, regardless of whether the transfer price is publicly observable, thereby constituting a subgame perfect Nash equilibrium (SPNE). However, we demonstrate that direct costing can strategically dominate absorption costing when one of the two firms is an incumbent, whereas the other is a potential entrant. Stated differently, the well-known result in the strategic cost allocation problem reverses if we consider entry threats. More specifically, we show that if the incumbent credibly commits to an observable transfer price, the upfront adoption of a direct costing system enables the incumbent to deter the entry of the potential rival in the SPNE. As a commitment device for the observable price, we consider the regulation of transfer prices that usually exists in oligopolistic network industries. We show that a regulator that pursues social welfare maximization approves direct costing but not absorption costing. Therefore, the firms and the regulator can share a mutual interest in the adoption of a direct costing system, a state thus sustained as the SPNE. This result yields managerial accounting implications for a divisionalized firm facing the threat of potential competitors entering the market in that the firm can use this accounting system to help monopolize the market.