I've taken a tour. AR$60. They have 3 a day in English. Depending on the day they are either at 11am, 1pm, & 2pm or... 12pm, 1pm, & 2pm.
They sell about 1/4 of the tickets online and then the rest can be purchased in person. So you may think that a tour is sold out, but it's really that they aren't offering any more tickets for online purchases.
Bring your camera, because you can take photos during the start and at the end of the tour when you enter the main concert hall. You can't take pictures during the middle of the tour probably so that you will buy souvenir books, but also because a 50min-60min tour would stretch into a 90 minute tour in a snap.
At the Colon there are two box offices. One for buying tickets to shows and one for buying tickets to tours.
Tours in Spanish run every 15 minutes. And for some reason, I have no idea why, but they have yet to add tours in Portuguese despite the fact that about 30% of all of the people in the lobby seem to be Brazilian whenever I duck my head in there. But I would expect tours in Portuguese to be coming soon.