add_action('wpcf7_before_send_mail', 'TotalPrice');

(function TotalPrice()
{
    //Select checkboxes by id
    var $checkboxes = $('input[id=checkboxprice]');
    //Set event
    $checkboxes.on('change', function ()
    {
        var totalPrice = 0;
        $checkboxes.each(function()
        {
           //Sum values of checkboxes
           if(this.checked)
               totalPrice = totalPrice + parseInt(this.value);
        });
        $("#price").val(totalPrice);
    });
})(jQuery);