How do I display a custom field from a custom taxonomy in single.php?

Catchable fatal error: Object of class stdClass could not be converted
to string in /wp-content/themes/new/single.php on line 22

This is because $wpdb->get_row returns an object by default and you can’t echo an object. Changing echo $toc to echo $toc->meta_value will give you your desired result.

Your other problem is that multiple terms can have have the same term_id. Your custom table should be using the term_taxonomy_id which will always be unique.

global $wpdb;
$term = get_term_by('id', $termID, 'dataset');  //Seems redundant does $term->term_id === $termID???
$query = $wpdb->prepare( "SELECT * FROM {$wpdb->taxonomymeta}
                          WHERE `taxonomy_id` = %d
                          AND `meta_key` LIKE 'baseurl' ",
                          $term->term_id );
$toc = $wpdb->get_row( $query );
echo $toc->meta_value;