It's interesting the way you drew the opposite conclusion that I would. I look at that and think "$50 a year to route a few packets? To heck with that."

Part of that is surely that I don't see what makes having a very ordinary Linux box sitting around making noise a hobby. To each his own.

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.

"a watt for a year is about a buck" (fifty). Thanks for that.

A one-year payback time means an IRR (hope I'm using that term right!) of 100%. If you're choosing between putting your money into paying down your mortgage at 6%, investing in the stock market at 3% (plus or minus enormous volatility), insulating your house at 20%, or replacing your old PC router with a cheap MIPS box at 100%, go for the cheap MIPS box!

That's assuming you paid $0 for hardware, and your time is worth nothing.

No, it takes the cost of the router into account. The cost of the now-turned-off PC is immaterial; it's a sunk cost. (Although maybe freeing it up from router duty will make it more valuable since you can, e.g., sell it.)

It's true that it doesn't take the value of your time into account.