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.