This module recalculates the sell price of the line item via Rules and Ajax on Add to Cart form.
Enable module and create your own rule based on the supplied example rules file (commerce_line_item_ajax_recalculate.rules.example).
Shane Graham (https://www.drupal.org/u/turbogeek)
This module was created from community patches, see: https://www.drupal.org/node/1541886
Thanks to the patch authors: Andy Giles (https://www.drupal.org/u/andyg5000) Josh Miller (https://www.drupal.org/u/joshmiller)