Ubercart alter product price when product added to cart
Here is another hook you can play with hook_uc_add_to_cart

Parameters:
$nid - the node ID of the product
$qty - the quantity being added
$data - the data array, including attributes and model number adjustments


Here is how to work with hooks


  Drupal's module system is based on the concept of "hooks". A hook is a
  PHP function that is named foo_bar(), where "foo" is the name of the
  module (whose filename is thus foo.module) and "bar" is the name of
  the hook. Each hook has a defined set of parameters and a specified
  result type.


function hook_uc_cart_item($op, &amp;$item) { the work hook should be replaced with your modules name like 

function YOURMODULENAME_uc_cart_item($op, &amp;$item)

See Hooks