Change product price before putting it on a cart
You want to alter the cart item, not the product node. hook_uc_cart_alter is what you're looking for. 

function MYMODULE_uc_cart_alter(&amp;$items) {
  foreach($items as $key =&gt; $item) { //Set all items to $3.50
    $item-&gt;price = 3.50;
  }
}