The Shopping cart and the product view page in magento

Question

I know the location of the prouduct page is view.phtml and calculate price in /varien/Product.js , Abstract.php for custom option prices' labels. (I had made some modifcation for these files)

But what about the shopping cart's? i know default.phtml is about the frontend and get price from Data.php but what about the custom option values? Does it reload the price like Product.js ?

I want to standardize the prices in my magento but they having different custom option price.

So for example consider i have offer a half price discount for the product so (karat tone : 18k +37.5$) in my product page but (Karat tone : 18k +70$) appear in my shopping cart (no discount) The product price is fine in my case, only the custom option has problem

You only need to tell me where does the custom option of the shopping cart local.

None of these paths helps i presume its in the backend, inside mage or so observer file.