How can a user reasonably project data usage before buying a new iPad?
Addressing your query head on - several US carriers have very nice planning tools:


Verizon Wireless Data Calculator
AT&amp;T Wireless Data Calculator


Both are quite close to what I'm seeing for my usage patterns (one calculates 2.5 GB and the other 3.15 GB) in practice.

I would say to take these with a grain of salt since with iOS 7 - you now have exceptionally good information on exactly which app used exactly how much data. That coupled with automatic data alerts that you can configure from the carrier to alert you at whatever levels you want, you can easily shut off parts of cellular data (or all cellular data) to shape your usage without having to downgrade your cellular service.

In effect, iOS 7 makes it easy to steer your usage to fit your budget if and when you find yourself using more data than planned.

Also, in the US - you are not locked into any level of data as the service is month to month. I often have months where I tether 2 or 4 GB extra data for business reasons. I have a shared data plan between my phone and iPad so a quick online session upgrades that month to the next higher tier of data. I set an alarm to undo that change at the end of my bill cycle and there are no fees to increase and decrease - just fees for the data (and overage should you go there for a month). For me, the extra $10 for a month is far less than the overage of $15 per GB and simpler for me to manage when bursts or overages happen (I'm looking at you iTunes match streaming all night on an iPad while in my bag with the smart cover closed :/