I have an iPhone with 6GB of data a month and if I'm not tethering it I can't come close to using that much.

I purchased a data plan for the states (my 6GB of data is Canada only) and got 75MB worth. In 8 days of fairly heavy use (browsing, google maps mainly) I used just over 75MB.

The main places I use data at home are public transit to make an hour long bus ride more tolerable. Almost everywhere else I have Wifi or other access. You may want to consider when you think you would use data (IE bus, train, hotel rooms) and that may help you decide if you need 1GB of data.