Yes, more expensive - although IMHO drives are pretty cheap these days and the OP could use the original drive in an external case for backup purposes. More backups are always good.
And Green drives, at least the WD Caviar Green that I use, don't just "slow down", they actually stop when left idle for a while. My 1TB Caviar Green drive uses just 0.4W of power in standby mode.
At any rate, if anyone else is looking at this thread for the purposes of making a buying decision, I'm convinced that a Green drive is definitely the way to go to fill this need.