We used (N,V,T) Monte Carlo simulations to calculate the configurational entropy and the free energy of two charged disks neutralized by mono- and divalent counterions. All ion-ion, ion-disk, and disk-disk interactions are described in the framework of the Primitive model. This treatment includes interionic correlations responsible for long-range attraction between colloids under high electrostatic coupling conditions. The free energy is fitted by an empirical law which may be used for numerical simulation of colloidal suspensions in the framework of the One Component Plasma theory, including electrostatic and excluded volume effects.