Scotland Yard confirmed it is costing £11,000 (Rs 9.3 lakh) every day to ensure the Australian does not flee his bolthole at the Ecuadorean Embassy.
The final bill could be much more as the 41-year-old continues to defy extradition to Sweden where he is suspected of sexually assaulting two women.
Officers have been watching the property in Knightsbridge, west London, since Assange breached his bail and claimed asylum in June.
London Mayor Boris Johnson confirmed the bill between June 20 and September 10 was £905,000. If the costs continued at the average of £11,000 a day the total would now be over £1.1 million.