I was looking into the possibility of mocking up a 3D city in UDK and came across the following links on youtube.

Google Earth is an abundant source of low res 3D models for any given city.  
And you can freely download most of these models as .skp files.
Once you have a .skp file from Google Earth, you can load it into Google Sketchup Pro (a free trial version is available for up to 8 hours of dev time).
After that you can export that .skp file into a .fbx file that UDK will recognize, including textures.
Although to work with the original textures you will also need a batch texture conversion tool, like IrfanView.
Its a bit of a process to work through, but becomes easy enough the more you do it.
Check out these videos for more detail.

How to Import SketchUp Assets/Models to UDK


How to import Materials/Textures from SketchUp to UDK

Here is a Google Earth model I converted to the UDK platform. 


Unless you are only interested in exporting 1 Google Earth at a time, there are more efficient approaches to exporting larger groups of buildings in one shot.

Exporting Google Earth Models into 3DS Max

Part 1 – Exporting Google Earth Models into UDK

Part 2 – Exporting Google Earth Models into UDK