Display Custom Price and Tax information

functions.php

<?php

// Add a custom field to the General tab on the product edit page
add_action( 'woocommerce_product_options_general_product_data', 'add_custom_price_option' );
function add_custom_price_option() {
	woocommerce_wp_text_input(
		array(
			'id'          => '_custom_price_title',
			'label'       => __( 'Display Price', 'your-textdomain' ),
			'description' => __( 'Enter a custom price title to display on the product page.', 'your-textdomain' ),
			'desc_tip'    => true,
			'type'        => 'text',
			'placeholder' => __( 'e.g., Special Offer: $99', 'your-textdomain' ),
		)
	);
	woocommerce_wp_text_input(
		array(
			'id'          => '_custom_vat_title',
			'label'       => __( 'Tax text', 'your-textdomain'),
			'description' => __( 'Enter a custom tax text to display on the product page.', 'your-textdomain' ),
			'desc_tip'    => true,
			'type'        => 'text',
			'placeholder' => __( 'e.g., p.p. excluding VAT (21%)', 'your-textdomain' ),
		)
	);
}

// Save the custom field value when the product is saved
add_action( 'woocommerce_process_product_meta', 'save_custom_price_option' );
function save_custom_price_option( $post_id ) {
	$custom_price_title = isset( $_POST['_custom_price_title'] ) ? sanitize_text_field( $_POST['_custom_price_title'] ) : '';
	update_post_meta( $post_id, '_custom_price_title', $custom_price_title );

	$custom_price_title = isset( $_POST['_custom_vat_title'] ) ? sanitize_text_field( $_POST['_custom_vat_title'] ) : '';
	update_post_meta( $post_id, '_custom_vat_title', $custom_price_title );
}

// Display the custom price title on the product page
add_filter( 'woocommerce_get_price_html', 'display_custom_price_title', 10, 2 );
function display_custom_price_title( $price, $product ) {
	if ( is_product() ) {
		$custom_price = get_post_meta( $product->get_id(), '_custom_price_title', true );
		if ( ! empty( $custom_price ) ) {
			$tax_text = get_post_meta( $product->get_id(), '_custom_vat_title', true );
			if ( !empty( $tax_text ) ) {
				$custom_price .= '&nbsp;<small style="font-size:50%">' . $tax_text . '</small>';
			}
			return $custom_price;
		}
	}

	return $price;
}