My mirror clocks in at 21 Gb. I'm self hosting so there's no real money cost besides the one-time investment in hardware. A really chap person could probably host this off a raspberry pi attached to something like a Corsair Voyager and serve multiple requests per second.
There's some technical trickiness in actually storing the files in a file system, since most filesystems don't deal well with having millions of files in the same folder. So I've had to build a four-tier directory structure based on the hash of the file name to be able to store the files. They're stored in a structure like 31/444/781/225/foobar.gz.
I did some experiments storing them as BLOBs in a database, but I couldn't get it to work.
The bandwidth usage is pretty inconsequential since most of my pages clock in at just a few kilobytes.
If you want to look up sources it's better to use the live wikipedia at any rate, since it will be more up to date than this snapshot.