Cloud computing replaces large upfront expenses with low, variable payments that only apply to what you use. CFD simulations cover a range of sizes and complexity. Activity is generally interrupted by quiet periods when simulations are halted, e.g. when a simulation ends non-working hours, during analysis of results or preparation of a new simulation, etc. The fluctuating demand makes CFD well suited to a pay-per-use model. Users need to understand the costs involved to get the best value from cloud and be confident that they fall within budget. This article explains the costs, with an example of an external aerodynamics calculation with OpenFOAM using CFD Direct From the Cloud™ (CFDDFC) on Amazon Web Services (AWS).