I just called two companies and they both had fixed prices regardless of my location.
Premium (5238-0000) was ~AR$80
Ciudad (4923-7007) was ~AR$75
I got an English-speaking rep on the phone fairly easily (if you don't feel confident with your Castellano). The Premium rep seemed ready to make my reservation straight off, but the Ciudad rep told me to wait 30 minutes prior to when I want to leave and then call them.
I personally might just take a taxi to Manuel Tienda Leon's bus station. My taxi from Congreso will probably be AR$10, and MTL's website says their EZE bus is $40 so this is slightly more hassle but cheaper. Pros/cons to everything.
And obviously, if you hate your life, yes you could take the colectivo.