At the current electricity prices we're paying here (~$0.35/kWh), this would be around 4 W of continuous consumption. That's not a lot, you can basically power

At the current electricity prices we're paying here (~$0.35/kWh), this would be around 4 W of continuous consumption. That's not a lot, you can basically power you Pi with it, but not the screen... count at least 10x more costs to operate the screen 24h a day. But as mentioned by someone, if you use a sensor to only turn on the screen when there's someone in front of it (and if you actually turn OFF your screen when there's nobody, and not just display a black screen), then you can almost remove the display power from the equation, unless you spend your day in front of the mirror.