Heh, as a kid I always wondered about this. My parents hated if someone left the lights on, but my computer has a 600w power supply... how many bulbs is that!
Unless you were cranking everything to the max 24/7 (like maxing out your GPU while reading/writing to a database while doing a bunch of floating point calcs for something?), it probably idled at around 100 watts. Not great, but not something I'd worry about.
What matter more is whether flicking the switch wears out the components more than letting it run. The replacement cost matters more than the running electrical cost.