The notion of cognitive miserliness that people can automatically perceive and sensitively react to cognitive cost in cognitive task completion has been accepted widely. No research has investigated the underlying neural processes associated with consumers' cognitive miserliness. We developed a paradigm for event-related potential (ERP) using online purchase decision-making tasks with high/low cognitive demand to explore consumers' neural responses to different levels of cognitive cost. Participants faced either deliberative (Deliberative) or heuristic decision-making (Heuristic_DM) in each trial. Four ERP components explain consumers' neural responses to cognitive cost. First, Deliberative_DM evoked larger feedback-related negativity (FRN) than Heuristic_DM. FRN can be evoked by anticipated negative outcomes. The larger FRN might mainly reflect the participants' loss perception led by anticipated high cognitive effort in Deliberative_DM. This finding might also imply that consumers encode cognitive effort as a cognitive cost with negative utility. Second, a larger P2 was evoked in Heuristic_DM than in Deliberative_DM, suggesting that more attentional resources were allocated to the stimuli leading to Heuristic_DM demanding less cognitive expenditure. Third, P3b was observed in Heur-istic_DM, while slow-wave (SW) was observed in Deliberative_DM. Observed P3b, related to target stimulus classification, revealed that Heuristic_DM with low cognitive cost might be categorized as expected target stimuli. The SW, an ERP indicator of mental calculation, demonstrated that Deliberative_DM launched an effortful mental arithmetic process. Our insights will help marketing professionals better understand consumers' responses to the cognitive cost of information processing in purchase decision-making.