Okay I'll try to give you a few headers to give you some options. First, if you are willing to root your device (you can always unroot after the trip): you can install droidwall. Insuring you know exactly which apps have access to your 3G data and aren't draining your usage.
I've found 3G watchdog to be the most accurate data counter out there, and you would not have to "set it up". It's already set up! Just maybe reset the counter once you get off the plane
Also, for my monthly rates on emails (I do not download pictures, so I get solely text versions of the emails) I use about 20mb/month. That's probably receiving about 20 emails a day and sending about 10-15 emails a day. So In total I'd say 1000-1500 emails (cause the replies can get very big) is about 20mb of data. But keep in mind I use push emailing, and I only poll my network every 60 minutes which uses about 100kb every time I poll (I have two email accounts set up).
Also, browsing the web can really drain the data usage. I make sure I use Opera Mini because I disable pictures from loading, and it compresses the webpages by up to 80%. So I'd say each webpage is about 10-20kb so in 20mb you could view about 1300 webpages using my method.
Then again if you'd like pictures, keep in mind each picture is about 20kb minimum and can get very large.
I hope you find some of this information useful!