For some reason, I never collected traffic statistics of my internet
connection, even though I could have easily done that for years now.

Well, I started doing it in October. Two months are complete now and it
looks like this (RX, TX in gigabyte):

    2020-11:  148.43   25.14
    2020-12:  206.21   24.81

This is way less than I expected, considering all the video streaming
and Arch Linux's updates. But then again, I don't really know. Is that a
lot? Is it not?

For mobile access, you pay something like 50 EUR per month for 50 GB
here in Germany. Well, uhm. Good thing I have a landline, I guess?