From my experience in New York, most of Lower Manhattan, at least, is flat in elevation, even though satellites mistake the terrain as being hilly due to the large number of skyscrapers...
(similar effects can also be found in Tokyo)
Oh yeah, Gijs, I think these maps might be helpful to manually create elevation data, if you want to go that way. 'hope this can be of some use!
Source 1
Source 2
Elevation of Manhattan.
Simulated original terrain of Manhattan of the 1600s.
Difference between present-day and former terrain of Manhattan. (Black/Gray = Increase in Elevation, Brown/Red = Decrease)