In Denmark, what happens when you reach the limit (on private plans[1]) is that your speed is limited to something between 64-256kbit/sec depending on the provider. You will never be financially penalized for going over.

What is also true, however, is that most basic plans include between 10 and 20 GB of data. You have a really bad plan if you only get 3 GB.

[1]The rules are a bit different for company plans.