Here we offer a O2 locked internet stick for prepaid sim-cards.
And this are the prices for mobile internet within Germany:
1 day flatrate: 3.50 EURO
1 month flatrate: 25.00 EURO (please do not forget to cancel this option because it will prolongated otherwise)
Have a look to my weekflat offer for this data sim card: You can buy a weekflat code via my shop for this item.