Android Data Usage

On a foreign prepaid simcard, i would not give the phone full access to the bandwidth budget. This to prevent having no bandwidth left once you really need it.

Next to that having it on "online" standby also drains the battery. For example, a simple app like WhatsApp is known to drain the battery switching between (3G) Cellular and WIFI when either one is not available.

I'd recommend measuring your daily "idle" data usage in your home country. Without it's no way to predict the bandwidth you need on a weekly basis in "idle" mode.

Here's a cheap example of a prepaid internet package in Europe (Greece). More expensive packages can easily cost double / triple this amount.

Get an app called netcounter, counts all your data use per day week, month, as you like.

Put a wifi and net toggle switch on your home screen and with a simple one touch you are in charge when your phone connects and when not. Useful for power saving as well.

BTW, Check your your roaming cost, in particular data roaming. Can cost you an arm and a leg, some country can cost over 10$ per MB.
the 'trouble' getting a local prepaid sim is worth it, usually. Just had been a week in Laos, sim card plus 200MB data for grand total of 2.75$...

So far this month, I have used 37MB of data on my LG Sol. I pay 1 Mexican Peso per MB (.08 usd). so the cost is not great. Turn off automatic updates as most apps are more than 1MB and several updates can add up. Beware of streaming videos and music. Mucho megabytes there. Turn off notices from Facebook, Twitter, etc.

Thanks everyone for the advice. I guess one of the largest contributers to data when the phone is "idle" are the automatic app updates. Will try out the netcounter app and will see how it can assist my data usage.

Lets hope that disabling automatic updates will help stop my data from running out even when I'm not using my phone.

Yes, I disable automatic updates too, then only do it when connected to wifi.

regarding automatic updates, at least on my android, it is possible to select "only when connected through wifi", or something similar.
I use 3Gwatchdog to account for my data usage, which helps a lot and actually lets you select a limit that would supposedly cut off data access from apps (although i have never reached that point).
In any case, as my data roaming is really expensive, I automatically turn the data access off when in roaming (also an option in the settings of the phone), and only use internet access through available wifi connections when traveling.