Kim, Novemsky, and Dhar (2013) ran this gum experiment in South Korea. Participants were given W1,000 (about $1) and asked which gum they would like to buy. Participants could also choose to keep the money and not buy any gum. When both gums were priced at W630 only 46% of participants decided to buy one of both options, but when the price was slightly different (W620 vs. W640) this proportion increased to 77%.
Excerpt from: The Psychology of Price: How to use price to increase demand, profit and customer satisfaction by Leigh Caldwell