Some back of the envelope calculations:
Length of US/Canada border: 8,890km [1] Population of Canada: 36,708,083 [2]
Assuming ALL of Canada's population lives within 100km of this border a rough estimate of this area might be 889,000km^2 giving a population density of 41 people per square kilometre. This conservative estimate is in the same ballpark as the US population density of 33 people per square kilometre [3] with the caveat that this population is spread across a long, thin 9000 km ribbon increasing the cost of roads and any other infrastructure versus the US's more favourable shape. And let's not forget that Canada's GDP per capita is $46,437 while America's is $57,467 [4]. This is a factor of 1.24 in favour of the United States.
Given all this, why is the US incapable of providing the kind of single-payer healthcare that Canada does to its population?
[1] https://www.statcan.gc.ca/pub/11-402-x/2012000/chap/geo/geo-...
[2] http://www.statcan.gc.ca/tables-tableaux/sum-som/l01/cst01/f...
[3] https://en.wikipedia.org/wiki/List_of_countries_and_dependen...
[4] https://en.wikipedia.org/wiki/List_of_countries_by_GDP_(PPP)...
The rest of the country isn't paying for a thing in NYC - it's the other way around. Just leave us more of our own tax money and we could do all sorts of things that aren't currently considered possible.
At (roughly) what number of people does that argument stop working? 5M? 500K? 50K? 5?
You make a good point about policing. Density wouldn't explain why policing in Tokyo is better and cheaper than in New York.