I got a Drei sim card in Vienna at €9.90. My classmate got his in Mediamart in Buhl Centre, a mall in Krems, for less than €4. Feeling terribly cheated as usual. I thought this sort of thing only happened in Southeast Asia. I guess not.
Anyway, this gives you 1GB, 1000 free SMS's and a couple of other standard things for 15 days. Thereafter, you pay online on their website using your number and a PIN provided on the card your sim card comes with, and you get 30 days more of roughly the same. I think it goes from 1GB to 1.5GB at that point. I forget.
If you can wait, purchase it in Krems rather than Vienna. If you plan your day well, start it early, you can visit the thrift shop close to the Buhl Centre as well. The address is: Second-Hand Laden d Caritas, Sankt-Paul-Gasse 12, 3500 Krems an der Donau.
It closes at 4pm. I think it opens at 8am. Anyway, go early. It does close at 4pm. It's got tons of great stuff for super cheap. I got all my kitchen utensils there. You can get winter clothing there too.