When offering a product for sale, presenting the sale using multiples of a number that allow the consumer to follow your mathematical logic with ease results in the consumer choosing that product over one with an obscure price, according to research by King and Janiszewski (2011).
Charging $40 for a gift basket in which the customer chooses 2 $20 items or 5 $8 items proves more successful than charging $40 for that gift basket and providing non-numerical information (Gift baskets: $40! Several Items included!). Including the number is an important visual aid that leads the consumer down this particular mental path.
Read the deal and calculate the per-unit price yourself to decide if you are really getting a good deal.