Depends on where you're getting it from and how it is. An average "Cheap" cake is going to be in the ¥2,000 range and they're....
Fine.
I mean go with a chocolate one if you're going cheap because at least the level of sugar will mask the lower quality, cheap shortcakes are just super bland.
Expensive cakes can run anywhere from ¥5,000~20,000 (And honestly, it's hit or miss on if they're even good or not, I got a 10,000 cake for my birthday last year from my roommate, you could tell it was quality because the whip was really good, real heavy cream, very fresh tasting, but the shortcake was just... egh.)
((Ok I guess I just don't really like strawberry shortcake to begin with, it's just too bland for me, when I make it I load on extra strawberries and add powdered sugar to the middle icing to just give it some added sweetness, but storebought shortcake no matter where they're from are like eating a can of whipping cream, it's just not good.))
But even outside of Christmas season this is your general range of what cakes cost.