"Ein Käsesandwich ist zusätzlich."
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?