Currently there's a very good offer, which I've actually ordered for myself yesterday.
There's an eBay seller, who will pay out € 200 in cash and give you a Nokia 6300 + a 3G-USB-modem for free, if you subscribe the O2 Internet Pack L (3G flatrate) with him:
http://cgi.ebay.de/O2-Active-Data-In...d=p3286.c0.m14
The flatrate itself costs € 25/month or - if you can proove to be a tradesman - € 20/month.
So the total costs of the flatrate during the 2 years of contract duration will be 24 x € 25 = € 600 or 24 x € 20 = € 480 if you can get the tradesmen-discount.
But you get a cash-discount of € 200 plus the phone and the USB-modem, which have a value of another € 200. So in the end you have an effective total of € 200 or € 80 for two years of mobile internet, which equals to € 8.33/month or € 3.33/month.
However you should first check if there's 3G coverage by O2 in the places, you aim to use the flatrate. If not, you could go for a Vodafone-based flatrate für € 30, which is offered by
Moobicent or take the mp3.de mobile prepaid variant.
O2 and Vodafone provide HSDPA on their 3G network, while eplus has only plain UMTS. Outside 3G-coverage only Vodafone has currently nationwide EDGE-coverage. O2 and eplus do support GPRS, but O2 announced a nationwide EDGE-rollout during the next months.