Tversky & Kahneman (1981) reported that most participants decided to drive when they could save money on a low-price good as compared to when they could save on a high-price good, even though the discount prices were same. Although this irrational decision making has been interpreted as a rate-dependent estimation of value (prospect theory), this study newly proposes that it can be explained by the certainty of purchase based on the price of goods. Experiment 1 replicated the previously reported difference in decision making, and additionally demonstrated that participants’ certainty of purchase was lower for a high- than a low-price good. When it was emphasized that participants’ intention to purchase high- and low-price goods were equally sure, decision making did not significantly differ (Experiment 2). Furthermore, decision making differed based only on the certainty of purchase even when prices of goods were same (Experiment 3). Consumers’ decision making may be rather rational, depending straightforwardly on the certainty of purchase that is susceptible to price.