Alter dynamically line item price on cart (drupal commerce)
Have you read any of the product pricing rules documentation or watched the tutorials on DrupalCommerce.org? It's all in there: http://www.drupalcommerce.org/user-guide/product-pricing-rules-screencasts

In other words, you shouldn't be hooking into anything, you should be using Drupal Commerce's core product pricing rules system. This is the only supported way to make consistent line item unit price alterations that are reflected in the final order total.