Measurements of the mass function of galaxy clusters provide tight constraints on various cosmological parameters. However, in order to fully exploit this powerful probe it is necessary to develop accurate models for the scaling relations between the total cluster mass and its observable properties. In this note I describe a semi-analytic approach to modeling the hierarchical evolution of galaxy clusters, which allows to estimate mass-observable relations and their scatter. This model is computationally efficient, and will be particularly useful in the analysis of current and upcoming cluster surveys.