E.g., see "Bob" in Austria: http://www.bob.at/gigabob
The table (for one of their plans) says:
Included volume per month: 9 GB
Cost per month: 8.80 Euro
Each additional GB: 4 Euro
Fee to activate the plan: 0 Euro
Max. speed: 4 Mbit/s
Commitment period: None
I'd say this is pretty transparent to the user and there aren't really any "traps" in the plan (except that traffic gets a little bit more expensive once you are past 9 GB in a given month).