I've been doing some checks over the last few days. I've used about 20mb in 4 days. This is made up of residual background updates, overnight ONLY. During the day I'm using Wifi and have used, in 4 days, about 550MB made up mostly of email, browsing, the news application, checking for updates in the apps store, downing a few small apps (typically 1Mb each, x about 5), checking facebook, using twitter (a little), watching 3-4 YouTube videos (about 4-5 minutes each).
Almost everything is disabled. facebook and twitter only update when I go in to them, as does email. About the only thing which updates in the background, every 3 hours, is the weather apps. Stocks don't update unless I go in to it - which I don't.
I've also solved the problem of low-storage (messages) as I had 948 facebook contacts listed under contacts. You have phone contacts and if you slide to the right you will see flickr, facebook and twitter contacts. At some point when I was fiddling about I enabled twitter in contacts, and it synced the details of the people I was following. This meant contacts was taking up about 38MB. I cleared all contacts (Applications-> clear data) and this disabled the twitter automatic logon in contacts (deleted my username/password) and then I put back the contacts from syncing to my PC (Outlook). It was a bit of a fiddle as only some of my contacts wanted to sync down to the mobile. I went in and modified the remaining contacts and they have now synced. (This is an HTC Desire btw). When you "Clear Data" on contacts it also clears login info for facebook and twitter, but not HTC's email app, or K9. There is also several places in the applications where you need to explictely tell it not to sync, and especially tell twitter not to sync contacts.
Now I've gone from about 22MB free to 57MB free (see this using the MiniInfo app). Remember you'll need at least 25MB free to take the next update (working space needed during the update).
Wondered off the topic a bit, but the point would be that unless you stop all the background stuff syncing all the time, and you make sure you're contacts aren't trying to pull down details of your twitter followers, I think quite a few people may reach their 500MB limit quicker than they might be expecting.
The app I'm using to find our how much Wifi and 3G I'm using is called "Network Counter"