line-item-multiplier.php

<?php
add_action('woocommerce_order_item_add_action_buttons','wpo_bulk_edit_line_item_prices',10,1);
function wpo_bulk_edit_line_item_prices($order) {
	?>
	<input type="text" class="bulk-multiply-items-multiplier" size="5" style="float:left;"><button type="button" class="button bulk-multiply-items">multiply prices</button>
	<script>
	jQuery(function($) {
		$('.button.bulk-multiply-items').click(function(event){
			event.preventDefault();
			var multiplier = parseFloat( $('.bulk-multiply-items-multiplier').val() );
			$('.edit-order-item').each(function(){
				var type = $(this).closest('tbody').attr('id');
				if (type == 'order_line_items' || type == 'order_shipping_line_items') {
					$(this).trigger('click'); // enable edit mode
					if (type == 'order_line_items') {
						$line_subtotal = $(this).closest('tr').find('td.line_cost input.line_subtotal');
						$line_subtotal.val(parseFloat($line_subtotal.val())*multiplier);
					}
					$line_total = $(this).closest('tr').find('td.line_cost input.line_total');
					$line_total.val(parseFloat($line_total.val())*multiplier);
				}
			});
		});
	});
	</script>
	<?php
}