Relatively small. Compared to "war, disease or chaos", keeping "the area between the Mississippi and the Rockies" irrigated will be (is) expensive, but it's a relatively small expense.
Decent government. Stable institutions and mostly fair and equitable justice and law enforcement. The US and Europe have their challenges, to put it mildly, but have been fairly successful in averting chaos, and things are going pretty damn well, judging by even relatively recent historical standards.
> But how much do you spend to prevent war? How much do you dump into the CDC?
A lot. So that means that things can be very expensive and still be relatively cheap.
> And how do you irrigate the Midwest once the aquifers get low? Piping desalination water from the Gulf?
I don't know. Perhaps. Or perhaps redirecting water from the Hudson Bay drainage basin. Human ingenuity is pretty great.
But how much do you spend to prevent war? How much do you dump into the CDC? Is there a Bureau for the Prevention of Chaos?
And how do you irrigate the Midwest once the aquifers get low? Piping desalination water from the Gulf?