I just got back from a weekend in Germany. You cannot miss the huge posters everywhere proudly procaliming "€29 to anywhere in Germany, €39 to many destinations in Europe".
That price is called "Dauerspezial" (constant special rate). When you search for a connection on bahn.de and want to check the price, there is a column for the full price and another one for the special rate. I think you can book trains ahead something like 60 or 90 days. That is when the special rate becomes available as well. There is a number of tickets for each connection. And once that is sold out, it is sold out. So if you want the special rate, book early.
When there are two people, you can get the family variant of the Dauerspezial (you do not have to be family, just traveling together). We did that earlier this year with a friend. I think it was €49 for any train from anywhere to anywhere for both of us - so €24.50 per person, a further savings.
This rate is what makes many people in Germany use the trains a lot. I know I have not paid more than €29 in years. It really is available from anywhere to anywhere if you book early enough.