Nike don't need City for exposure. In the grand scheme of things, it's the other way around! Seriously, the Nike brand dwarfs the City brand.
Ideally they'd like to be using our superstar players to endorse their products, but most of them have boot deals with other companies.
Of course it's not 'about' shirt sales, but the shirt sales have a direct correlation to the price Nike will pay. The fee we get is calculated on shirt sales + value added by being associated with City. probably in a 70/30 ratio.
Another factor is additional merchandising - and if Nike have the rights to use the City brand on things like footballs, water bottles, general sportswear etc.
We've a long way to go before we start selling in the same quantities as Liverpool or United. That's not to say we might cut a better deal with a renegotiation or different partner, but shirt sales matter.
500,000 shirt sales is roughly 15 million extra revenue for a shirt manufacturer so 2 million shirts would be worth an extra 45 miilion - hell of a difference.