Yes, that's why. There's an uncommon site with recruitable trolls (
http://dom3.servegame.com/wiki/Troll_Pit). I believe that there's one that lets you recruit Troll Kings too, but I don't recall what the name of it is off-hand. I'm fairly certain it was used in the Preponderance game.
If you ever want to be certain about a particular unit, check the wiki or Edi's database. Anything that has a gold cost listed will cost upkeep.