I haven't tried it that direction. In Sept 2012, I bought I ticket from Prague to Bad Schandau. the first stop in Germany, for 357 Kc, the equivalent today of 12,98€. The Bahn sells a ticket from Bad Schandau to Prague today for 27,60€.
In Prague, they showed the price of the ticket in euro from Prague to Bad Schandau, but I don't know if that meant they would sell it in euro, or if that was just showing the equivalence. A few days earlier, in Zelezna Ruda, I bought a ticket to Prague using Kc that I had brought from Denver.
If you buy a ticket from the Bahn for Dresden to Decin, it's 16,60€. The Czech price from Decin to Prague is 180 Kc, about 7,15€. That's 23,75€. A Bahn ticket from Dresden to Prague is 34,60.
You don't say where you live, but in 2012 I got $40 in Czech Koruna to use for Czech tickets at the border from Wells Fargo for Interbank rate + 14%.