They have two data price plans:
- 24 ct per MByte OR
- 2,50 EUR per day (midnight till midnight) but only on days you'll use it:
Look here:
FONIC Surf-Stick - Internet-Tagesflatrate ohne Grundgebühr!
The tariff plan may be changed. The best way to do this is by e-mail.
Usually, the stand-alone sim are on the 24ct/MByte plan, with a USB-stick you get the day flatrate tariff (and a stick, which is unlocked).