" A simple way to figure out which discount frame seems larger is by using something called the Rule of 100. If the product’s price is less than $100, the Rule of 100 says that percentage discounts will seem larger. For a $30 T-shirt or a $15 entrée, even a $3 discount is still a relatively small number. But percentagewise (10 percent or 20 percent), that same discount looks much bigger. "

― Jonah Berger , Contagious: Why Things Catch On