Yep. I actually discussed this in a similar thread on HN a few days ago.
Fare revenue is approximately 3bn GBP. A 1:20 ratio.
In the worst case assuming no-one additional travels due to it being more pleasant, fares would have to increase ~5%.
My bus fare would go from 1.50 to 1.57.
This is a distraction, though. It's not relevant, because we shouldn't allow damaging activity to occur regardless of whether it might look marginally better economically if we zoom in far enough.
If the tube system funded itself by stealing wallets from random passengers, it would still be right to stop that program, even if fares went up, because it wouldn't be right or good.
A related concern I hear often is that if something becomes n% more expensive, then some people will be priced out.
The correct answer to 'poor people can't afford the bus' is not 'perform strange indirect forms of abuse on slightly less poor people'. The answer is 'fix the fact that people are poor'.
Public services should be funded via taxation, not by private advertising partnerships.