I got back from my first trip to Mexico City yesterday. Not sure if you have already left. The Archaeological museum (and almost all others) are closed on Monday as I disappointingly found out when I wanted to go to the Khalo museum.
The pyramids are open on Monday because that's where we went at the driver's suggestion instead of the Khalo museum.
It cost me a hundred bucks and change. I might have been ripped off but don't think so. He took me to the Pyramids then to the Basilica and a couple of other stops with a break for lunch. We left the hotel at 10 and were back around 4.
Personally if I had a short time in Mexico City, I would skip the pyramids. I found the City endlessly fascinating, vast and complex and certainly plenty to see, do and eat for 48 hours. But it's all a matter of taste and interest of course.