I would suggest the Econtel on Sommeringstraße. It has very reasonable pricing, the rooms are quite nice for what you pay (compared to what's in America, I'd say that they're nicer). It's only about two blocks from the Mierendorffplatz U-Bahn station. Assuming that you're not flying into schonefeld, it's only a stone's throw from Tegel.
The only downsides that I've ever come across there is that you cannot leave the building with your room key; it needs to be turned into the front desk. However, you'll never lose your key and thus never need to pay to replace it. Additionally, they don't do a very good job with early wakeup calls. I mean like really early... like before 4am. If you've got an early flight, don't count on them to be calling your room to wake you. However, I've never had them miss a call after 4.