Custome php code function addition in my wordpress ecommerce website

hi, i want to add a function in my wordpress ecommerce website. i want to have a function in my quantity option in product page, i want to have different prices for different quantities, for example a customer buy 1 quantity for my product then it should have a different price but if it added 2 quantity of a same product then function should append different prices for 1+1 quantity. i want to have for example 1 rupees for 1 quantity, and 15 rs for 1+1 quantity, so then the total price should be 16 rs not 2rs which is default wordpress option.