Neil,
It is going to vary depending on several factors, most of which you can control.
If you go the tethering approach, several of the carriers will send you alerts if you are nearing your pre-purchase so that you can buy-up.
Everything you do to reduce the data flowing back and forth will reduce the data usage; screen size, color depth, Aero effects, desktop background, other applications (esp. web browsing and email).
