Well there is also the bit of time at first power up where a pc, well really just about all electrical devices do, that it pulls up to 150% of it's normal power within the first... 2-5 seconds.

You may notice this with cheaper light bulbs, they come on REALLY bright for a second and then level out.

But really how much more will that cost you? Maybe a few cents per month.

Also need to take into consideration, boot time. For you, the computer is unable to be used durring the boot process. So, in a way, it's wasted time and electricity. If your pc takes (counting boot up and shut down times) 1-2 mins between being useable and off... figure in how many times per day you reboot. Lets say 5. That's 5-10mins easy of wasted time and energy.

Do that every day, well you get the idea.

I leave mine on all the time. I usually don't shut them down unless I'm going away for more than 3 days.

Monitor I'll turn off if I'm not going to be at the pc for more than 1hour. No screen saver needed. Waste of cpu cycles and just encourages you to leave the monitor on, using even more electricity and wasted cpu useage.