Well, it's not as easy as you make it sound.
For example, in our paypal integration, we give to paypal the unit price of products and their quantity and paypal calculates the total on its end. And there is no way to tell paypal to calculate it differently. So that would mean that you would have to change the paypal plugin code to give the full price without the order details if you want to use paypal. Then, you have that modification when loading the cart but also for the cart and order display.
The main code responsible for the calculus of the qauntity price, is in the files currency.php (funciton quantityPrices) and order.php (function loadFullOrder ) of the folder administrator/components/com_hikashop/classes/