How to set a default value in a meta box

Here’s a tweaked version of the referenced wpse103469_add_price_per_unit_meta_to_price() function. If no units have been specified, the default units are appended to the price, otherwise the user-specified units are used.

add_filter('woocommerce_get_price_html', 'wpse103469_add_price_per_unit_meta_to_price');
function wpse103469_add_price_per_unit_meta_to_price( $price ) {
     $default_units =  __( 'sq. ft.', 'myplugin_textdomain' );
     $units = get_post_meta( get_the_ID(), 'wc_price_per_unit_key', true );

     return ( $units ) ? $price .= ' ' . $units : $price .= ' ' . $default_units;
}