Sadiq Khan's plan to widen London's Ultra Low Emission Zone is set to cost up to £130m, according to City Hall.
The bill includes a network of 750 new enforcement cameras, each costing between £10,000 and £15,000.
Owners of older vehicles with engines which fail to meet modern emissions standards face a daily charge of £12.50 to drive in the zone.
The ULEZ will expand from central London to the North and South Circular roads on October 25th to honour an election promise by the mayor.
Transport for London said the expanded ULEZ would pay for itself.
"It is expected that when the zone is expanded up to, but not including, the North and South circular harmful NOx emissions from vehicles will fall by around 30 per cent across the capital," said Alex Williams, TfL’s Director of City Planning.
Emma Best, Conservative member, London Assembly added: "There’s a real risk that the cost of expanding ULEZ could run even higher. Many Londoners will only make short journeys within the zone, and if the Mayor plans to catch them all, he may need even more cameras. At up to £15,000 a site, the bill could quickly add up. So even if Londoners avoid the charge, their taxes will end up paying for it anyway.London Assembly Tories have called on the mayor use a £50m reserve fund to reopen two 'scrappage schemes' to help drivers struggling to afford a new car.