That fundamentally changes if I can counter your Times Square billboard with a direct mail campaign in Minnesota.
And you can have Nebraska. The whole state, I wouldn't run a single ad there for a national campaign, in either scenario. Why would I? The ad money I might have to spend in Nebraska to get a thousand eyeballs might get me closer to ten thousand eyeballs in California or New York. Not to mention the ten thousand eyeballs I get in New York are also eyeballs I want to see my general platform and the fact that my party exists, because as you may well know, most people vote a straight ticket.
I see a lot of compelling reasons to believe that local media would remain a primary means of communication between candidates and prospective voters, even if we switched to a popular vote.
What market force are you imagining makes ad space cheaper in wealthier states?