I have 7 running 24/7 - 4 of them dont have monitors and are accessed via VNC. It costs me £20 (thats UK sterling) extra per month what that would be in your currency i wouldn't know.
If you do run your machine 24/7, make sure your power saving options turn the monitor off after a selected time. (But obviously not system suspend or standby

)
Another way of looking at it, is every time you switch your pc on/off, you will get spikes in the electric, sooner or later something will blow. If it is on 24/7, this can't happen.