The conduction-band minimum energy V-0 (relative to vacuum) of quasifree excess electrons and positrons in fluid rare gases, methane, and silane is calculated as a function of fluid density n. The calculations are performed within the framework of the Wigner-Seitz mean-field approximation for nonpolar fluids, using a simple "core-tail" pseudopotential to model the excess electron (positron)-atom/molecule interactions. This pseudopotential is determined only by the atomic/molecular polarizability and the electron (positron) scattering length. Despite the simplicity of our approach, the present V-0(n) results, which contain no adjustable parameters, show close agreement with those obtained using more elaborate potentials and with available experimental data.