Is video compression worthwhile?
A 4.7 GB (or actually 4.4 GB) DVD-R disk costs roughly 50 cents (without media tax) in large spindle boxes, or approximately 11 cents per GB, and therefore by compressing I save some 12 cents per hour of movie. Large USB or Firewire disks cost roughly 80 cents per GB, so when I store to them, I save some 88 cents per hour of movie. These hard disks suffer another drawback: I can't insert them into the (DivX-capable) family DVD player and watch the movies on the big screen.
Compression takes quite a bit of CPU time, roughly four hours per one hour of movie on my machine. I estimate my machine consumes roughly 60 watts more electricity when the CPU is working. Electricity costs roughly 8 cents per hour of kilowatt (including taxes and transfer but no fixed costs), or roughly two cents per one our of movie.
So yes, for all current media, my video compression is indeed economically justified, albeit not necessarily indefinitely: in future media will be cheaper and power more expensive.
There's another argument: space. With individual cases I can store roughly 200 DVD disks per a meter of shelf space, up to 15 shelves high. That's 3000 disks. They cost 1500 euros, a shelf might cost some 300 euros, but the floor space in the center of Helsinki costs currently a whopping 4000 euros per square meter. This quadruples the benefit of video compression!
Now there's just one aspect left to ponder: The full shelf can store 33000 hours of compressed movies. Assuming I watched four hours of movies every day - which I definitely don't have time to do - it will take over 22 years to watch all the disks. Furthermore, since my current recording speed seems to be well below one disk per week, filling the shelf would be concluded by my potential grand children. But by then the copyright to the movies has extinguished and my grandchildren could legally share the movies (or actually largely also documentaries) I have recorded.