I've only had it since Friday, but what I did was I loaded a 300mb bundle from Vodacom, as soon as I got it, i've now used about 90mb's, but that is because of all the downloading of apps, and also seeing how the browser works and so on. Now that I know how all the stuff works, and I've stopped downloading and browsing around at every chance I get, my data usage has dropped dramatically.
Remember that you can customize this phone a whole lot, so you can pretty much choose how much data it is going to use, but to be safe, I'd say allow 10mb/day if you do no other browsing of downloading, and just let the phone get periodic updates etc. Obviously this will vary heavily depending on what updates you ask it for, and what accounts you choose to sync, so please don't take this as written in stone...I'm not paying any ones data bills, LOL.
Some things to try would be:
Turn off background downloads: This can help with battery a little, but also save some data. Bare in mind that market needs this enabled, but if you select market while it is off, it'll take you to a screen to re enable it.
Customize your account sync options: You can tell the various accounts how often to sync, or if you would rather sync them manually. Obviously, you'll use less data syncing manually, or by letting them sync every 6 hours instead of every 1 hour.
I was using 3G watchdog at first, and it does a great job of keeping track of your usage, but I found that it was sometimes preventing my phone from entering sleep mode, and so it was eating the battery up. Another app that did this was Task Killer(the one with the red man as the icon), and system manager free....I have now uninstalled them, because a lot of people on various forums say that task killers are not essential.
I have Juice Defender running, and also Powersave Mode. These seem to work very well. I also downloaded Sysmonitor, and this does a great job telling me about RAM, SD Card, Processor and Battery. Before removing those other apps, System Manager Free or what ever it is called, reported that at idle my CPU was almost always clocked above 90 % and sometimes up to 100%...This is obviously no good for the battery.
After removing the apps that I mentioned, and installing the new ones, I now have an idle CPU usage of about 12 -16% at most, and I'm happy! I don't really need 3G watchdog, as Vodacom4me gives me a fairly good idea of the data usage, and so if it is getting close to empty on there, I can always just load a small top up or something.
Hope someone will find this useful, cos the CPU usage debacle I had was really frustrating since this is my first android phone. I was on the verge of factory resetting, but then decided to try fix the problem myself, and it seems to have worked. I took my phone off charge at 6pm yesterday, and as of now, it still has about 78% battery left. This includes phonecalls, mails, sms etc. So my current battery usage is +/- 1.5% per hour. Theoretically that means my batter could last as long as 60 Hours on a charge at my current usage. It probably won't but even if it gets to 48 hours, I'll be happy!
Don't worry, in the first few days the battery life drastically improves, by day 6 or 7 it has just about normalized. If you could have made it more than a day in the beginning, I am sure 2-3 days would be a standard expectancy. I bought a car charger for mine, Body Glove. It works well, and has the whole usb setup going as well, so if I get stuck somewhere, I can always charge via pc anyways.
