D-networks do not have to be expensive: Congstar (a Telekom subsidiary) and Fyve (a product of ProSiebenSat.1 Media AG) are two low-cost providers who operate in the Telekom (Congstar) and Vodafone networks respectively.
Fyve is a classic prepaid provider, whereas Congstar also has postpaid contracts as well as DSL and VDSL in its product range. Both offer calls and SMS for 9 cents/unit each as well as data transmission at up to 7.2 Mbit/s. So: Fyve or Congstar?
I've been with Congstar and I have not regretted it. Thanks to the telecom network, Congstar is really very well developed, and the prices are still competitive. At Fyve I would miss the opportunity, to use LTEEven though it is limited by the bandwidth, it offers better pings and thus faster Internet than a comparable UMTS connection.
The prices differ only in the details: for example Congstar 1 GB of data volume 12.90 Euro, with Fyve you will be charged 15 Euro. If you need less, you can also use 500 MB packages (for 9.90 at Congstar and 10 Euro at Fyve) - even smaller data packages are available, but in my experience they are too small for serious use on smartphones. The complete prices and offers can be found on the respective websites of Congstar or Fyve. Both providers offer Nano-SIMs for the iPhone 5.
Which of the two providers to choose depends on the level of coverage in the areas where you spend most time. The telecom network offers a few advantages: complete EDGE expansion (so that you can at least pick up emails quickly even in areas without UMTS) as well as LTE support for the iPhone 5 - although the rate is reduced to 7.2 Mbps even when using LTE. The advantages such as a low ping and (so far) still low utilization of the radio cells by a few LTE devices remain, however.
Apart from Congstar and Fyve, there are of course other discounters, but they are all on a similar price level and have the same conditions as far as the network is concerned. For discount providers in the so-called e-networks (even if this classification is no longer clean and other frequencies are used) like Fonic or Simyo the offers in the "Premium" networks are a hard nut to crack. I regret at least my Change from o² to Congstar not.