Infrastructure as a Service clouds are a flexible and fast way to obtain (virtual) resources as demand varies. Grids, on the other hand, are middleware platforms able to combine resources from different administrative domains for tasks ex- ecution. Clouds can be used as providers of devices such as virtual machines by grids so they only use the resources they need at every moment, but this requires grids to be able to decide when to allocate and release those resources. Here we analyze by simulations an economic approach to set resource prices and resolve when to scale resources depending on the users demand. Our simula- tor is based on the well-known GridSim software for grid simulation, which we expand. The results show how the proposed system can successfully adapt the amount of allocated resources to the demand, while at the same time ensuring that resources are fairly shared among users.