"Ein Käsesandwich ist zusätzlich."
A cheese sandwich is in addition (an alternative translation) is awkward English
the German sentence is silly. Who says that? I wrote "A cheese sandwich is more", one could also write, "is extra"
I don't really know what the German sentence really means, but "A cheese sandwich is more" makes no sense. That said, the translation I got, "A cheese sandwich is additional", isn't doing much better in the 'sounding like normal english' department.
It makes sense when put in a certain situation. Say you come to a student's cantina, and you order some specific menu, but want to add cheese sandwich thinking the menu, without something plus the sandwich, would cost the same money as the specific menu itself. The answer you coul get is :"One/A cheese sandwich is additional", meaning you need to pay it extra.
That's how I see it.
No, the answer you get would be 'Das Käsesandwich ist inklusive', if it's included or 'Das Käsesandwich kostet extra', if it isn't included. Anotherone: Ich möchte zusätzlich noch ein Käsesandwich. On top of what you already ordered, you want an additional sandwich. The combination of 'zusätzlich sein' is very uncommon. 'zusätzlich' needs a reference – on top of what, in addition to what?
In that case the waitress would just say "it will be $1.00 extra" or "it will cost you $1.00 more", or something like that.
Alternatively, they could write on the menu something like "add cheese for $1.00", such as they usually do on Subway menus.