The most appropriate way in my opinion ni kufanya benchmarking kiasi kwanza from people doing the same business as you. Unaeza enda ucheck vile mtu anauza different types of books(go as a buyer) ask as much questions about different books and prices and if necessary jokingly ask why the prices differ, is it the size? For novels is it the author? Is it the publisher? For drinks, ukiendea stock angalia umeuziwa how much let's say umeuziwa a certain drink(B) at a wholesale prices of 45kshs, go to few supermarkets and see how much they sell drink B, go to shops and ask how much drink B is too, then put a price that will favor some few profits and wont hurt the customers' pocket.