Probably not more then $50.00 a month... however I read somewhere that replacing all the incandescent bulbs in your home with the energy saving compact fluorescent bulbs can help off-set the price increase.
These bulbs are a lot more expensive then the cheapie incandescent ones but they last WAY longer and use less energy. A regular 60 WATT incandescent bulb is replaced with a 13 Watt Compact Fluorescent Bulb. Plus good for the environment.
I have no idea if this is actually true or not... but since I'm waiting for my seeds to arrive anyway ... I've replaced all 57 indoor incandescent bulbs in my house with the Compact Fluorescent Bulbs. I'm anxious to see how much (if at all) my electric bill drops (I have no MH or HPS lights on right now).