The ability to weigh a reward against the cost of acquiring it is critical for decision-making. While the medial prefrontal cortex (mPFC) has been implicated in tracking both mental effort demands and net value of rewards, these findings primarily come from choice paradigms that confound increased task difficulty with decreased reward probability. To resolve this issue, we designed novel tasks that kept probability of success – and therefore probability of reward – constant between levels of effort demand. In two experiments, participants completed a novel effort-based reward task that manipulated effort demand and either reward magnitude or probability of success. Electroencephalogram (EEG) data was recorded to compare an electrophysiological index of mPFC function (frontal midline theta (FMT)) to an index of incentive salience (component P3) at both cue evaluation and feedback phases. We found no evidence that FMT tracked effort demands or net value during cue evaluation. At feedback, however, FMT power was enhanced for high compared to low effort trials, but not modulated by reward magnitude or probability. Conversely, P3 was sensitive to reward magnitude and probability at both cue and feedback phases and only integrated expended effort costs at feedback, such that P3 amplitudes continued to scale with reward magnitude and probability but were also increased for high compared to low effort reward feedback. These findings suggest that, in the absence of option comparison and unequal likelihood of success, the mPFC does not track net value of prospective effort-based rewards. Instead, expended cognitive effort potentiates FMT power and enhances the saliency of rewards at feedback.