That is an extremely high cost of electricity.
I live in what I believe to be a relatively high electrical cost area and I pulled up my bill online for 2/13/2013 to 3/14/2013 and I'm paying 13 cents per KWh solely for energy although by the time I add on the substantial fixed monthly meter fee, the state low income assistance tax, 100% energy for tomorrow (in theory, all my KWh come from the local windfarm instead of from coal, in practice its probably merely a greenwashing scam) I'm writing a check (well, paying online) for about 17 cents per KWh, other words the number of KHw divided by the debit to my bank account. So the optiplex would cost me a whopping $4.04 per month lets call it a buck a week. Do I get a buck a week of fun out of my homemade firewall/PBX/other things? Yes.
If I did my math right, this is equivalent to about 4 minutes of labor at my current family income, other words spending time on detailed monthly accounting is more expensive than just paying for it outta the slush fund.
I don't have a wifi network installed merely to roast my brain with microwatts of RF. It exists solely as background infrastructure for a small herd of apple idevices and android phones/tablets all of which are value engineered to be disposable after a year or two. If I had no wifi devices I probably wouldn't have a wifi router. In other words if I wanted to save money in the category of "tablets", I'd look first at not replacing it every year or two. Just the capital/depreciation cost of only one wifi connected idevice is about an order of magnitude more than I'll pay for the electricity to run my home router/PBX/Buncha-other things. Electricity is so cheap its not even a rounding error in total systems cost, and optimizing for the wrong value is always a fail.
Another interesting anecdote is a couple decades ago I was taught as a pretty crude consumer product engineering estimate "a watt for a year is about a buck" but via inflation etc its now about $1.50. Apparently folks in less civilized areas are paying around $2 for a watt for a year. So something that runs 24 hours a day and costs $8 at walmart like a 5 watt clock radio alarm clock uses its own cost in electricity in a bit more than a year. This is also the genesis of trying to save money on wall warts, if a wall wart costs $2.50 and uses $5 of electricity per year, a more efficient switcher that costs $10 and uses only $1 of electricity per year pays for itself rather quickly.