In order to accurately use Radar REMPI as a diagnostic technique, care must be taken to consider how to extrapolate the electron and neutral atom number densities from a given signal. We investigate the dependence of the Radar REMPI signal on the electron collision frequency, specifically in xenon-helium (Xe:He) mixtures, where helium is the buffer gas and the concentration of xenon is 10 ppm or less. We develop a 0-dimensional kinetic model with REMPI ionization to determine the effects of the collision frequency on the signal amplitude and determine a quantitative map between the signal obtained and the neutral xenon density. Good agreement is shown for 1 ppm xenon in atmospheric pressure (∼760 Torr) helium, indicating the importance of modeling the plasma parameters to more accurately assess the signal. An upper bound for the cross-section of two-photon excitation from the ground 5p6 1S0 state to the 5p56p[5/2]2 state of xenon by circularly-polarized light is computed.