Business simulations are a powerful way to provide experiential learning that is focussed, controlled, and concentrated. Inherent in any simulation, however, are numerous assumptions that determine feedback, and hence the lessons learnt. In this conceptual paper we describe some common cost assumptions that are implicit in simulation design and discuss the implications for the lessons that are learnt from the simulation. In particular, concerns are raised about misconceptions that may arise when the assumptions are not recognised. Examples are drawn from a popular business simulation. We conclude that, while there are advantages from both simple and sophisticated approaches to costing, the impact on profits can be huge. When the assumptions are not explicit, they can send signals about cost behaviour which are inconsistent with reality. We recommend that when using a business simulation the facilitator explicitly recognise the assumptions, and thereby recognise the generalisations that can, or can't be drawn from the simulated experience.