CODE EXAMPLE FOR PHP

update cart subtotal woocommerce

add_action( 'woocommerce_before_calculate_totals', 'add_custom_price', 10, 1);
function add_custom_price( $cart_object ) {

    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return;

    if ( did_action( 'woocommerce_before_calculate_totals' ) >= 2 )
        return;

    foreach ( $cart_object->get_cart() as $cart_item ) {
        ## Price calculation ##
        $price = $cart_item['data']->price * 12;

        ## Set the price with WooCommerce compatibility ##
        if ( version_compare( WC_VERSION, '3.0', '<' ) ) {
            $cart_item['data']->price = $price; // Before WC 3.0
        } else {
            $cart_item['data']->set_price( $price ); // WC 3.0+
        }
    }
}