Published studies suggest that bypass therapy assay testing can be used to predict treatment response and dosing requirements for haemophilia patients with inhibitors. The aim of this study was to evaluate the costs of utilizing and not utilizing bypass therapy assay testing before treating mild-to-moderate bleeding episodes on-demand in haemophilia patients with inhibitors from a US third-party payer perspective. In our exploratory decision tree model, the average patient was assumed to be an adult weighing 75 kg. Based on existing head-to-head clinical trials, the efficacy of activated prothrombin complex (aPCC) and recombinant factor VIIa (rFVIIa) was assumed to be equivalent and based on expert opinion of the haematologist in our study it was conservatively assumed that assay testing improves the efficacy of both the bypassing agents by 10%. Probabilistic and one-way sensitivity analyses were used to determine the robustness of the results. Cost savings per bleeding episode were estimated at $6886 (95% CI = $4310–7978) for aPCC and $7647 (95% CI = $3134–10 388) for rFVIIa treatment. This translates in potential cost savings of 24.8% (95% CI = 15.5–28.8%) for aPCC use and 18.2% (95% CI = 8–24.7%) for rFVIIa use. Furthermore, if testing successfully predicts the optimum dose for concomitant therapy at the onset of bleeding, significant cost savings were observed compared with rFVIIa and aPCC therapies alone. Use of bypass therapy assay testing before treatment administration in haemophilia inhibitor patients can potentially reduce treatment costs significantly while optimizing dose and therapy response.