How much should I charge for cleaning an office?

This is a very dirty office that will keep getting dirty because it is a scale service business. Its not really big but cluttered and will take me a long time the first time to clean it. They want me to clean it once a mth. So with it being once a mth it probably will take just as long as the first time. I am not sure how much I should charge. I was thinking 200-300 a mth. Is this to much?