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To date I've not seen a lot of spec's on how much data is typically used by Android. A lot depends on what apps you use. For example, Google maps tends to use a lot of data transferring map info to the phone. Pandora radio and Youtube also tends to use a lot of data.
One way to get around a lot of the data usage issues is to set up the phone to WiFi networks when at known locations. My Droid X, for example, automatically connects to WiFi at my home and at my office.