The two major factors affecting the cost of operating a fermentation chamber will be the ambient temperature and how well the chamber is insulated. Obviously, the size of the chamber will also be a factor, but those are the main ones. When the ambient temps are anywhere in the vicinity of room temperature, ie say from 60-80 deg F, the cost to run the chamber at 60 deg F +/- will be very low. Probably only a few dollars a month at the most. Taking it down to lagering temps will cost more, but even then it shouldn't be more than about $6 or so per month. I'm basing this on some monitoring I did recently on the small chest freezer I use for serving. I was surprised at how efficient it was and how little power it consumed. Working against very high or very low ambient temps will run up the costs considerably, but even then it's not very expensive to run these things. The newer units are a lot more efficient than the older models.