okay to make a long story short, i have fiddled with numbers and come to a conclusion: all of the challenges are 25 levels more difficult than the same travel option from the city's location. as an example, for a city in burrow, the City -> Ealing challenge is 25 difficulty levels higher than the Burrow -> Ealing challenge

additionally, travel to the city's location seems to have a 50 difficulty flat base.

i will let it fall to somebody else to determine how to display this information on the wiki, though!