My searching efforts have failed when trying to find this. On average, how much energy does the raspberry pi consume in 24 hours ( minimal usage vs max usage in a day AND usb vs micro usb powered)?
So, yeah, probably not more than:
4 W*24 h = 96 Wh (I guess that's 345.6 kJ) (note this is energy, not a power)
and likely not less than half that if you get the power down to 2 W. This is for a Raspberry Pi model B, the model A consumes even less power.
Even at relatively high electricity prices you are looking at < $0.02 per day of running the box.
Note, power consumption has been changing (for the better) with improvements in the software, because it's possible for the OS to power down some blocks of the machine when they are idle or perhaps if explicitly turned off.
Updated in Sept 2014 to note the release of the model B+: Less power! According to measurements by Alex Eames (see http://raspi.tv/2014/how-much-less-power-does-the-raspberry-pi-b-use-than-the-old-model-b )
A lightly-loaded model B with keyboard = 1.89 W -> daily 45.36 Wh
The B+ really offers huge improvements in the power circuitry. Wow!
Also please realize that although it's very little power, if you are buying a solar panel, battery, charge regulator, etc. just to run the pi, you are spending a lot more on power hardware than you are spending on computing.
Regarding shutting off USB/LAN i/o for sleep/processing and then re-enabling it, see Disable LAN9512