Used to be around 3- 5 Gigs because I'd be constantly streaming spotify.  I actually still do, but Tmobile no longer counts music against your limit as long as the app registers and qualifies with tmobile.
Then it dropped down to about 2.5 gigs per month.  I do web browsing, youtube, some light games, and team speak, surfing the AT forums, and GPS navigation mostly.
I'd think 200MB would be terrible to deal with.
That said, one thing that you should note is that the more capable your phone is, the more data it will use.  A lot of that is because the more capable your phone, the more you will use it for everything around you.
I had a BB9000 that I loved for the keyboard, but it was a PITA for anything beyond text.  I ended up installing opera browser, and then killing all images because all it took were a few images to make my experience terrible...so I got used to reading walls of text.  Walls of text don't amount to that much data 
