I understand their desire for more revenue and somewhat agree with selling advertising space (as much as I hate that sample), I think that specific advertising space is worth a lot more than $84,000 per year.
From a recent article:
$84,000 is perhaps the total cost of one driver, or based on the above about 750 hours of service for one bus. It's a drop in the bucket for the budget. It is not enough money to make any difference, but the impact on people is.... well, I think it would be quite annoying.
I don't know how much ad-space normally goes for, but I do know that a TV commercial is millions. McDonalds can afford to pay more in order to put a picture of a Big Mac on a card that I must have in my wallet and look at every day.