I got my iPhone 4 repaired last week on the 5th floor of A世界 which is across the street from the main computer city building. Prices were cheap, I paid 100 yuan to fix the proximity sensor. Fixing broken screens should be in the 250-350 range for an iPhone 4 or 4S. On that floor there are two shops specifically for repairing Apple products, they’re easy to find by the white illuminated signs with the Apple logo that say 维修.
I actually have a phone number for the place that I went: 028-85028405 and cell phone: 13438936901
Taobao: http://cdym.taobao.com