How much data is transferred using Andreas Schildbach's Bitcoin Wallet Android app?
Since January 14 on my Galaxy Nexus, Bitcoin Wallet for Android has used 120 bytes of mobile data. It's used approximately 160 MB of data on WiFi in the same period. One month cycle earlier, it used 463 KB of mobile data, and approximately 113 MB on WiFi. I plug it in every night and whenever I'm in my car. 

I can't answer for the amount of data necessary for the initial synchronization, other than advise you to do it on WiFi for both speed and data limit reasons.