These sort of phones should only be used with a "unlimited" type of data plan, there's no way to estimate what you will use.

I've used between around 700 megs to a gig per month on my 3G unlimited plan and HTC Hero (Android) phone in the few past months. This is with streaming music (Spotify), RSS, Facebooking, web pages, GoogleMaps for navigation, some youtube etc...pretty normal use, I would say except for the streaming music which might not be that common.

Anyway, my point is that the only way to use phones like this is to have a set price - and to make sure data roaming is off if you leave the country/area