That's good as it will be a discount from the £12.50 charge you have to pay today to drive a non compliant vehicle in the ULEZ.
This whole 'he said, she said' thing sounds a bit fishy. I wouldn't be surprised if someone somewhere with a vendetta against the Mayor is making numbers up without actually thinking it through, and I also wouldn't be surprised if it eventually turns into policy.
I looked into train driving as a career change a while ago, and it's incredibly difficult to get through the selection process. The 'interview' is a 3 day process, with lots of testing, and a very high drop out rate. Then there's classroom training, with many exams that weed out a lot more before they even get behind the controls of a simulator. It takes 18 months to get qualified. For every thousand that get as far as the interview only a handful get through, and many thousands more don't even make it to the interview stage.
So it becomes a balancing act between the complexity of the process to get qualified, and the wages offered. As the industry already has problems getting enough skilled train drivers at wages significantly higher than £35k, the only way to lower wages would be to make it easier to get qualified, which would impact safety and reliability.
But where's the money coming from to make it significantly easier or cheaper? If someone has got to pay for it, who better than the people doing the behaviour that they want to stop?