Ice Queen's suggestion is good.
And I didn't take standby mode into account because the OP said it would be left on all the time for SETI@Home.
$200 is what a "typical" computer plus CRT would cost at 11 cents per kWh, averaging 240 watts of constant usage.
Shit, 11 cents per kWh? Expensive electricity! Thats almost twice what I pay.
Believe it or not, that's the US average, when I looked it up!
The site could have been wrong, though, or outdated.