Introduction: Cost studies are increasingly popular given resource constraints. While scholars stress the context-dependent nature of cost, and the importance of theory, cost studies remain context-blind and atheoretical. However, realist economic evaluation (REE) privileges context and the testing/refinement of economic programme theory. This preliminary REE serves to test and refine economic programme theory for supervision training programmes of different durations to better inform future programme design/implementation. Methods: Our preliminary REE unpacked how short (half-day) and extended (12 week) supervision training programmes in Victoria, Australia, produced costs and outcomes. We employed mixed methods: qualitative realist and quantitative cost methods. Economically optimised programme models were developed guided by identified cost-sensitive mechanisms and contexts. Results: As part of identified context-mechanism-outcome configurations (CMOCs) for both training programmes, we found a wider diversity of positive outcomes but greater costs for the extended programme (11 outcomes; AU$3069/learner) compared with the short programme (7 outcomes; $385/learner). We identified four shared cost-sensitive mechanisms for both programmes (training duration, learner protected time, learner engagement, and facilitator competence) and one shared cost-sensitive context (learners' supervisory experience). We developed two economically optimised programme models: (1) short programme for experienced supervisors facilitated by senior educators ($406/learner); and (2) extended programme for inexperienced supervisors facilitated by junior educators ($2104/learner). Discussion: Our initial economic programme theory was somewhat supported, refuted and refined. Results were partly consistent with previous research, but also extended it through unpacking cost-sensitive mechanisms and contexts. Although our preliminary REE fills a pressing gap in the methodology literature, conducting REE was challenging given our desire to integrate economic and realist analyses fully, and remain faithful to realist principles. Attention to training duration and experience levels of the facilitator–learner dyad may help to balance the cost and outcomes of training programmes.