drupal commerce calculator
If you already have a calculator that you like and the commerce portion is working the way you want it, the best thing to do is probably to change as little as possible. My first thought is to use a simple javascript that looks for the field where commerce stores the order total and copy it into your webform field. In my kickstart staging environment, there is a field called "ajax-item-price". So you could do a jquery one-liner like

$('#name-of-your-webform-field').val($('#ajax-item-price').html());