Replacement of a HDD is close to the $175 if you buy a 1TB. Plus there is time to reinstall and recover content.
I'd gladly pay the $175. But I take the option of $90.
Might I offer the observation that in all of your duly diligent "savings" computations, you've conveniently left out the cost of also running the monitor 24/7, which by some quirky coincidence, is the actual topic of this thread.
The only certain fact I can come up with with respect to powering down equipment involves duty cycle with respect to HID (High Intensity Discharge) lighting.
If you don't turn these light off, they will double their rated service lifespan. So, that means, if you run them at 12 hours per day, you only break even with bulb life, but double your cost of operation. Cold cathode lighting may or may not follow this trend.
As to your further nonsense with HDD life span, the WD160GB "Caviar" in the computer I'm using right now, is approaching 5 years old. It has been shut down thousands of times and It's still working perfectly. So, that HDD is worth about 40 bucks nowadays, and I'm sure that 5 years is plenty of return on investment.
It seems that when I'm talking to "service technicians" at this site, it's said that the results I'm getting can't be average, so I'm always dismissed as being "very lucky", since they know more about it than I do. Please keep in "mind" that knowledge is mostly theoretical, but performance is stone cold fact.
All that notwithstanding, I couldn't pick a lottery number if my life depended on it, and so it goes.
As to replacement cost of a 1 TB "green" HDD, here in the "other colonies", they are commonly available at the "$90.00" price range. Which incidentally, nicely coincides with the electricity costs that you tout as "savings" by not turning the drives off. (Here I'm speaking conceptually of course).
So, I wannbe so green, that's why I never power down my computer, still comes across as oxymoronic, hypocritical, and possibly outright stupid.
As a hedge against replacement of data issues due to failed drives, I maintain synced data across multiple computers, so that if a HDD actually does fail on me, all that need be done is a simple copy and paste operation to another drive.
As to *****s that don't make any provision for preservation of data, here again, it's your job as a tech to listen to their problems, sell them another drive, (parts & labor of course), then recommence your sobbing all the way to the bank.