The purpose of a CAZ is not to generate revenue but to improve the health of local residents by discouraging polluting journeys. We want to deter polluting journeys rather than raise money from them.
The money collected from a CAZ would go to central Government. The costs of running the central system would then be removed and what is left will be circulated to local authorities to cover the local operational costs of running the scheme. If there is any surplus revenue arising from the scheme, the revenue can only legally be re-invested to deliver improvements in local transport. So if there is any surplus money from the scheme it would be reinvested in the scheme itself and in sustainable travel options.