The Galeria Guemes rooftop view is the cheapest viewpoint in town. While prices fluctuate in pesos always, it's usually never more than a few dollars.
You'll go inside the galeria entering from Calle Florida. Stick to the right and find the Mitre elevator bank, take the elevator to the 14th floor. Here you can pay for your ticket and be ushered to the terrace view. Take your time to take in the 360 degree view.
Watch this Instagram reel for a play by play instruction on how to find it.
Check their Instagram page for hours, they typically close for an hour or two midday and don't operate on weekends or holidays.