Ab initio detailed calculations of the elastic properties of AgCl$_{x} Br$_{1−x}$ alloys were recently made using density-functional perturbation theory and by employing the virtual crystal approximation or by means of the full potential linearized augmented plane wave method. Here, we suggest a simple theoretical model that enables estimation of isothermal compressibility of these alloys in terms of the elastic data of end-members alone. The calculated values are in satisfactory agreement with the experimental ones. The present model makes use of an early suggestion that interconnects Gibbs energy for the formation and/or migration of defects in solids with bulk properties.