How can a user reasonably project data usage before buying a new iPad?

Apple announced the new iPad Mini models, and they look spectacular. I've never owned a device with a cellular data plan before, and the idea of near-ubiquitous connectivity sounds truly amazing. But buying a model locks you in to a particular carrier, and the carriers have very different pricing structures when it comes to data.

How would someone who has never used a device with a cellular data plan determine the amount of data they likely would need? Are there web sites with use cases of "this set of uses uses approximately this much data"? Ways to track data use from an iPod touch or MacBook Air to get a comparison? Without having used one of these before, what resources could help a consumer make an informed decision about how much data s/he is likely to consume before purchasing a device and thus locking into one carrier's pricing structure?

Addressing your query head on - several US carriers have very nice planning tools:

• Verizon Wireless Data Calculator
• AT&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 :/

It's very hard to track existing data usage, much less project future usage on a device that you haven't previously owned and are potentially just guessing how you will use it. Who knows what wild and wacky use cases you will find once you start using it?

Other iOS devices now track data usage from iOS7 onwards, but only for mobile data, not Wifi so you can't for example work out how much mail you were previously receiving on your iPod Touch over Wifi in order to take a guess at how much you may subsequently receive in the future over mobile data connections.

My biggest mobile data users over the last month has turned out to be Facebook > Podcasts > Safari > Mail in that order, but even this is not common usage, because during this period I spent time away with no Wifi on a social event that ended up with a lot of photo viewing/uploading on Facebook that I would normally do from home on a Mac, and forgetting to pre-download my podcasts for the several hours drive there and back...Luckily I had pre-loaded the map routing over WiFi so that was OK.

What I can do, again anecdotally, but perhaps more relevant, is know that I have only ever gone above 1Gb once since I got an iPhone, and I consider myself to be a relatively heavy user in some respects. I recently downgraded from an unlimited plan to a 1Gb one because it saved me a little money, and approximately half the time I don't even get above 250Mb per month.

As for pricing, I can't comment on US pricing structures, but in the UK, I can get an iPad from anywhere fully unlocked to all networks, and my own choice of carrier provides monthly data plans on a pay as you go basis from £5 for 500Mb, £7.50 for 1Gb and £12.50 for 3Gb, so if you were in the UK my advice would be to use a carrier with no contracts and choose what you might need each month based on your anticipated usage (If I am going on holiday, I simply won't buy one until I get home for example).

So the key answer is forget about trying to anticpate or judge what you might use, and get yourself an unlocked iPad, and a carrier that allows you to change your data plan to suit your immediate requirements rather than get locked into a 12 month contract.