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.
On the Raspberry Pi, you can pretty easily switch off the HDMI output via shell commands, which sends most displays into low-power standby. It takes a couple seconds to switch back on and wake the screen when activity is detected, but the tradeoff is easily worth it.
