Actually, center orchestra seats at the Met, equivalent to platea in the Colón are US$ 345 - considerably more than the Colón charges. The Met is so large that they discount side and rear orchestra seats down to $140, but these are in locations that would be in the plaza or across Tucuman in the Colón.
That said, I agree that seats in the lower levels of the Colón - those accessible through the main doors - are extremely expensive in pesos. What always surprises me is that they're fully sold, even though a pair of good seats costs the equivalent of a couple weeks' wages for the average Argentine.
If you get platea or palco seats to a "Gran Abono" performance of an opera at the Colón, you'll see the Argentine moneyed class like nowhere else I know. People come in from distant estancias dressed to the nines and wearing jewelry that they wouldn't dare show in the street. It's quite a show!
The cost of entry to the upper levels is extremely reasonable, btw, although these seats are very difficult to obtain. Sight lines are excellent in the more central sections. Even in the upper side galleries where you can't see much, the sound is incredible. The Colón has a clarity and immediacy of sound that I've never encountered in any other theater.