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;
}