I have a 400 watt HID and it only cost about a $10 a month increase in my bill on the 18/6 cycle. I changed out all my incandescents with CFLs before I started so I'm sure that offset the cost a bit (I have over 57 bulbs in my home).
Now... my portable A/C that I had to add to the room is another story.... that thing added about another $30 to my monthly bill.... totally worth it to keep my room nice and cool.