I live in NZ and until a couple years ago we (4 internet users) had a hard data cap of 60GB/month - for all of us combined - and were charged $1/GB or so extra over the limit (like Hodgman said above). It was hell, we almost always went over the limit and it was pretty much impossible to do anything other than browsing and downloading maybe 1 medium sized file per day.
We then moved to a different provider which offered (I believe) 100GB/month, which was easier, but with the catch that if you went over the limit, your speed was throttled down to sub-dialup speeds (we're talking about a few hundred bytes per second with regular interruptions every few minutes). This was easier to manage, but was a real pain in the ass every time we did go overcap, and even that was realistically not enough to download large files on a regular basis.
Finally we upgraded to 500GB/month and since then we haven't gone over the limit, settling on a total average of about 8-9GB a day with occasional spikes, which I find pretty reasonable (about 2GB/day each). Though one month we came really, really close to the data cap limit; still not sure what actually happened then
Data caps suck.