Advertising income was £130 million for last year [1], where passenger income was £4398 million[2], so around 3%. (Minus costs of running commercial advertising?)

I can see many people wouldn't want a 3% fare rise, but it shows smaller changes to the policy — like no longer having these whole-station or other excessive campaigns — wouldn't be a problem.

[1, PDF] https://content.tfl.gov.uk/tfl-advertising-annual-report-aug...

[2, PDF] https://board.tfl.gov.uk/documents/s19826/TfL%20Budget%20202...