My 400w at 24/7 costs about $34 a month. Your electricity rates change based upon region.
So yes there is a jump that is noticeable when adding a light. Now depending on your landlord is dependent on how he feels. If its winter the increase is understandable.
I hate landlords so I try and keep to the best with all of them. Be nice, pay on time, do extra things around the house. All of these can help you...just in case or as a preventative.
Back to your story, I would look up your regions rate and multiply and do the math. If you are worried about location do not post your rate on the forums = location. If your ballast is dimmable which if it is not I would replace it. For the reason that you can 250w one day and 400w the next with the flip of a switch. This will reduce that bill drastically. Other than that try and reduce other electrical pulls in the house. Also have you thought about exhausting that heat for your house to use? Carbon filter the end and voila you have a heater.