In Ubercart Which Hook is reponsible for adding product to the cart page(On clicking add-cart-button)?
If you want to alter the cart item, then you can use hook_uc_cart_alter of ubercart. 

This hook is not in documentation of ubercart 7 but its still working for Ubercart 7. 


  This is called every time the cart is rebuild (e.g. when products are
  added), so it's a good place to alter the cart item.


I have checked uc_coupon module and hook hook_uc_cart_alter is implemented into that module. So, you can try this hook.

Edit:
Code snippets to modify the price of item 

/**
 * Implements uc_cart_alter.
 */
function mymodule_uc_cart_alter(&amp;$items ) {

  foreach ($items as $index =&gt; $item) {

   //you can add condition for your bid product

    $items[$index]-&gt;price = 100;
  }
}


For reference:

http://www.ubercart.org/docs/api/hook_uc_cart_alter