I don't remember exactly how much the Atarazana was, but I seem to recall $70. Less than $80 for sure, and a small discount was given for paying cash.
I tend to hear the Colonial Zone referred to as the Zona Colonial, but the "ciudad" Colonial makes sense too. I'll leave the confirmation on that to the pros.
The Mercure is more central, being on Conde, but I liked the fact that being where we were a few blocks away from that area meant that we became familiar with a larger circle of the Zona. Crossing in front of the Alcazar de Colon twice a day, seeing it in all lighting and moods was a highlight. I have a picture of the Mercure Hotel at
http://travel.webshots.com/photo/278...64234328xmvHdv To look at the shots before and after, you can see where the hotel is situated.
Kaki