I don't know how much it costs to run a computer 24/7, but I do know that anyone claiming to save energy by leaving a TV on has some serious issues with reality. I remember being told that a TV was roughly the equivalent to a 500 Watt lightbulb, leaving a TV on will definitely run up an electric bill, as well as decreasing the useful life of the TV. I do know that a CRT monitor uses far more energy than the rest of the computer, so if you do leave your computer on 24/7, just turning off the monitor when it's not in use will save you on your electric bill.
Zenmervolt