I needed to buy a one-way ticket from Frankfurt to Milan. There is a super saver fare "Sparpreis" for only €79 in 2nd class that even includes seat reservations on some of the trains involved.
Now the bahn.com or bahn.de Website can only tell you wheter or not you got that fare after you entered all your booking and payment information and hit "I accept all the terms, please book now". No matter what connection I tried, it always came back as "fare sold out".
The available fare on my date of travel for the exact same connection was €179 one-way. I wasn't really prepared to pay that much.
Knowing that the shortest connections go through Basel I went to the SBB website and checked for Basel to Milan. Turns out they, too, sell a fare of €79 - and it was actually available. Basel has got three train stations, a German, a Swiss and a French one. Turns out that €79 fare was good from either one.
Frankfurt to Basel SBB (Swiss station) was €99 - but Frankfurt to Basel Bad Bf (German station) was only €49 on the bahn.com website - as that's considered a domestic ticket.
But www.ltur.de has an exclusive agreement with DB and can sell last-minute tickets (between a few days and about a month prior to travel) for extra low rates. Turns out, the exact same train that was €49 on bahn.com was €27.00 (+1.50 service fee) on http://www.ltur.com/de/bahn.html?omnin=DE-DB-BUEHNE-BAHN (if you change the German language flag to English you lose the link to the cheap tickets unfortunately).
So you have to be able to read German in order to book through ltur.de and "Bahn Restplätze" is otherwise a bit hidden on their website. But I ended up paying only €107.50 for the total trip - instead of €179.