Currently I get to around 400-500 MB per month with limited usage:
* 2 email accounts (private and work)
* checking the news (several apps) on the way to and from work
* checking the radar now and then (I like to know exactly when to expect rain)
No tethering, no Skype (via 3G), no Youtube (via 3G).
So if you’re planning on using Skype, you definitely don’t want to get stuck with 200 MB.
I have a 1 GB plan which allows me some freedom, but not much. Back in Israel I had unlimited which was way better, I loved the freedom it gave me to use my phone for anything I wanted without getting cut off or facing high costs.