If the men­tal cal­cu­la­tion required to deter­mine the dis­count given on a prod­uct is dif­fi­cult then we often mis­judge the mag­ni­tude of the reduction.

This “ease-of-computation” effect for judg­ing price reduc­tions is obvi­ously related to other recent stud­ies look­ing at ‘cog­ni­tive flu­ency’ and is another way to manip­u­late and be manip­u­lated through prod­uct pricing.

Con­sumers’ judge­ments of the mag­ni­tude of numer­i­cal dif­fer­ences are influ­enced by the ease of men­tal com­pu­ta­tions. The results from a set of exper­i­ments show that ease of com­pu­ta­tion can affect judg­ments of the mag­ni­tude of price dif­fer­ences, dis­count mag­ni­tudes, and brand choices. […] For exam­ple, when pre­sented with two pairs of num­bers, par­tic­i­pants incor­rectly judged the mag­ni­tude of the dif­fer­ence to be smaller for pairs with dif­fi­cult com­pu­ta­tions (e.g., 4.97 – 3.96, an arith­metic dif­fer­ence of 1.01) than for pairs with easy com­pu­ta­tions (e.g., 5.00 – 4.00, an arith­metic dif­fer­ence of 1.00).

via Bark­ing Up the Wrong Tree