It’s free! And really great! Parliament Building in Berlin is also known as Bundestag. There are 2 visits you can apply, the visit to Dome and the guided tour.

For both tours you have to apply in advance, they are free. Highly recommended is the guided tour, we booked 3 months in advance. Bring your passport with you to proof your ID and the print out. You’ll need to do security clearance.
For the guided tour, you can only take pictures, but no recording of voice, image etc. Our guide was excellent, highly informative and cared for the visitors. There were few small collections of arts, artifacts.  We got the chance to sit on the visitors bench inside the chamber. It was pretty impressive, important decisions are made here. 

Once done. We visited the Dome with audio guide. Awesome view! I think this was the highlight when in Berlin, I don’t think it’s well-known by visitors.