In that case it would be :

42mb per minute (1mb per minute per player, on a full server)

2520mb per hour

60480 per day (1 server)

423,360mb per day (7 servers).

Those would be the maximum though, if you're going on the premise that it costs 42mb a minute because its 1mb per minute per player. The servers aren't always completely full, so it would be a varying size.

Having said that, I have reviewed demos from other sites, and they are all around 20mb. That's SourceTV recorded ones, 20 minutes matches, with 20-40 players on the map.

You have to remember that everything that the demo runs on is already stored on your computer. All the textures and models are all in your cstrike folder, so the demo file is only code that just allows predetermined player movement to be rendered visually on your machine. It's not like screen capture, so I don't think it would be as big as you hypothesised. My sum at the top was only correcting your maths, as I feel that it was slightly flawed.