Create Prestashop custom function

Hi, We need to know how much is the cost for developing a function or module which adds automatically a complementary product and returns product to cart in prestashop when user adds certain product, example: The user adds product1 and cart shows product1 + its complementary product wiht the same quantity also cart shows (optional) the returns product with 0 quantity. If user sets 2 qtys then the price mus be -20 if sets qtys price must be -50. And go on. Any question we'll solve. Thanks.