So, the DB lists them as gold cost zero but it seems they do have an upkeep. It could definitely be me imagining things, as I haven't spent the time actually test it.
The DB values are correct. Checked them all by making the units recruitable, so it would have showed a gold cost. Wolves have always been upkeep free. Black hawks used to have gold cost in Dom2, but not in Dom3. Surprised me that it had been altered, but it's better this way.