98. A Board may make special provision for the cleansing of any factory, hotel, club or group of buildings or lands used for any one purpose and under one management, and may fix a special rate and the dates and other conditions for periodical payment thereof, which shall be determined by a written agreement with the person liable for the payment of the conservancy or scavenging tax in respect of such factory, hotel, club or group of buildings or lands: Provided that, in fixing the amount, proper regard shall be had to the probable cost to the Board of the services to be rendered.