A quick take on this
Powering a Pi say it uses 750mA @ 5V = 3.75W (My Pi2 runs a GPS, and SenseHAT and Camera and WiFi, taking pictures and GPS data every second writing to SD and taking data from the SenseHat as fast as it can, and writes when it has 100 data point (which is pretty quickly and runs easily from a 1A/5V USB Power bank.
Now that's converted from 230V so some power must be lost, let's call it 5W used (as it's an easy number
So at 5W for 24 hours using an average UK Electricity cost (15.5p/kWh Sept2015) = 1.86p per day
So for a year £6.80
Now electricity is cheaper now, even on a standard tariff now.
750mA is just a guess (I only know an A+ with SenseHat and GPS and WiFi was about a third of that iirc.)
I will measure it and measure at the plug too, with the official Pi3 power supply if waned or curiosity gets the better of me.
But I will say in the many 'yank the cable out of the Pi's' the kids/pupils have done there has never been a problem.
Now a nice button to turn it on and 'shut down' would be a good idea here, just because you know there will be a problem and a grumpy child involved, but honestly the Pi's (B+/A+/B2/B3/Zero we have don't seem to mind.)