Lauren, here is some info on prices that I have (in case you need). I just got bought 12 months at WellClub for 2400 pesos (that was cash price and for 2 people ie 4800 in total). Also made inquiries at Megatlon who offered a cool $4300 per person (cash, for 2 people with bonus 2 months). That was a platinum level membership (Alto Palermo etc). I believe that price is slightly higher now also.
A couple of months I got only a 1 month stint at WellClub and paid 360 pesos (cash). I am sure it is a little higher now though (although I dont know). Not sure of the Megatlon monthly price.
Most gyms also do 3 and 6 months depending on what your after.