add_filter('woocommerce_product_add_to_cart_text', 'custom_woocommerce_product_add_to_cart_text');

	function custom_woocommerce_product_add_to_cart_text($text) {
		global $product;
		if ($product->is_type('external')) {
			return __('See Prices', 'woocommerce');
		}
		return $text;
	}