Adding unit ammount after WooCommerce price. Example $ 24 /m2 and $24 / m

Firstly, doing this with CSS in my mind absolutely isn’t the way to go.

Secondly, paying 129$ for an extension to have this minor feature would just be ridiculous – nothing against the plugin per se.

Thirdly, below outlined basic solution is based on the example from the add_meta_box codex page. There are various other solutions for achieving this out there, just take a look around, this attends to get you started.

Code:

/**
 * Adds a box to the main column on the Post and Page edit screens.
 */
function wpse103469_wc_price_per_unit_mb() {

    $screens = array( 'post', 'page', 'product' );

    foreach ( $screens as $screen ) {

        add_meta_box(
            'wc_price_per_unit_mb',
            __( 'Price per Unit', 'myplugin_textdomain' ),
            'wpse103469_wc_price_per_unit_inner_mb',
            $screen,
            'advanced',
            'high'
        );
    }
}
add_action( 'add_meta_boxes', 'wpse103469_wc_price_per_unit_mb' );

/**
 * Prints the box content.
 * 
 * @param WP_Post $post The object for the current post/page.
 */
function wpse103469_wc_price_per_unit_inner_mb( $post ) {

  // Add an nonce field so we can check for it later.
  wp_nonce_field( 'wpse103469_wc_price_per_unit_inner_mb', 'wpse103469_wc_price_per_unit_inner_mb_nonce' );

  /*
   * Use get_post_meta() to retrieve an existing value
   * from the database and use the value for the form.
   */
  $value = get_post_meta( $post->ID, 'wc_price_per_unit_key', true );

  echo '<label for="wc_price_per_unit_field">';
       _e( "Price per Unit", 'myplugin_textdomain' );
  echo '</label> ';
  echo '<input type="text" id="wc_price_per_unit_field" name="wc_price_per_unit_field" value="' . esc_attr( $value ) . '" size="25" />';

}

/**
 * When the post is saved, saves our custom data.
 *
 * @param int $post_id The ID of the post being saved.
 */
function wpse103469_wc_price_per_unit_save_postdata( $post_id ) {

  /*
   * We need to verify this came from the our screen and with proper authorization,
   * because save_post can be triggered at other times.
   */

  // Check if our nonce is set.
  if ( ! isset( $_POST['wpse103469_wc_price_per_unit_inner_mb_nonce'] ) )
    return $post_id;

  $nonce = $_POST['wpse103469_wc_price_per_unit_inner_mb_nonce'];

  // Verify that the nonce is valid.
  if ( ! wp_verify_nonce( $nonce, 'wpse103469_wc_price_per_unit_inner_mb' ) )
      return $post_id;

  // If this is an autosave, our form has not been submitted, so we don't want to do anything.
  if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) 
      return $post_id;

  // Check the user's permissions.
  if ( 'page' == $_POST['post_type'] ) {

    if ( ! current_user_can( 'edit_page', $post_id ) )
        return $post_id;

  } else {

    if ( ! current_user_can( 'edit_post', $post_id ) )
        return $post_id;
  }

  /* OK, its safe for us to save the data now. */

  // Sanitize user input.
  $price_per_unit = sanitize_text_field( $_POST['wc_price_per_unit_field'] );

  // Update the meta field in the database.
  update_post_meta( $post_id, 'wc_price_per_unit_key', $price_per_unit );
}
add_action( 'save_post', 'wpse103469_wc_price_per_unit_save_postdata' );

The »Price per Unit« meta field can be outputted like this:

echo get_post_meta(get_the_ID(), 'wc_price_per_unit_key', true); 

in the according template file.

For adding the information from the »Price per Unit« meta field directly to the price output you can do something like this:

add_filter('woocommerce_get_price_html','wpse103469_add_price_per_unit_meta_to_price');
function wpse103469_add_price_per_unit_meta_to_price( $price ) {
    $price .= ' ' . get_post_meta(get_the_ID(), 'wc_price_per_unit_key', true);
    return $price;
}