The NAS I use is a Kurobox, it’s using 17 Watt, meaning about 150 kWh/a. In Germany, that’s about 28 Euros for non-stop power.
A good desktop machine might use about 100 Watt, which means an additional cost of 137 Euros. Two years, and the Kuro will have paid itself.
Might well be a MacMini is less power hungry than the above 100 Watt (on the other hand a server might have even more, see mas), and energy in the US for example is cheaper than here.
edit: from Apple site on the MacMini: Maximum continuous power: 110W
Hrm. So maybe the nslu2 with the spun-down drives is still the best answer, if power (or temp!) is your biggest concern.