They wouldn't buy the advertising if they couldn't make a profit on your attention. Pay with brain space, or pay through a tax. Which is more expensive?
It's not the brain space that's paying for the advertising, it's your (or, well, people's) buying of the advertised products. So in addition to the costs of brain space, there are still also actual financial costs to it that are borne by the city's citizens.
Well they got the product they bought, so it's a stretch to consider it a cost. Perhaps only the marginal cost that went to advertising, but it's still a stretch to consider it a cost if the buyer considers they got their money's worth.