Ticket to ride: What is the math behind the point values of city pairs?
The point value of tickets, as far as I can tell, in the minimum number of trains needed to link the two cities. 

I wont explain all of them as you can check your self but here are a few examples to show what I mean.

Denver to El Paso ticket is 4 points.  The shortest route is Denver to Santa Fe (2) to El Paso (2) totaling 4.

Boston to Miami ticket is 12 Points.  The shortest route is Boston to New York (2) to Washington (2) to Raleigh (2) to Charlston (2) to Miami (4) which totals 12.

Seatle to New York is 22 points.  The shorted route is Seatle to Helena (6) to Dululth (6) to Toronto (6) to Pittsburgh (2) to New York (2) totaling 22.

This works as far as I can tell for all other tickets and in my experience all other TTR games.  There may be a few exceptions but if designing your own map then this seems to be the standard formula.