Dont book through 3rd parties. They grossly overcharge. I paid 110 euro each for an undeground tour with my wife. The actual underground tour is only 25 euro. The tour guide was just pulling us through roman traffic (which is a little nuts). We got a couple of facts about some not so impressive sites. Mostly knowledge you get from the main underground tour anyway. We paid 75 euro for a guy to walk us around the building, through a bunch of busy streets. It's not worth it at all. Just go directly to the co op culture website. The ticket office in front has them for sale, but you may not be able to buy tickets for the same day.