We have used smash cakes as long as I can remember! My son had one, 11 years ago...

I do them for free, with the 1st birthday cakes. I usually do a 1 layer 6" or a 4" depending on how much batter I have left.
If they request a different flavor, I charge them $5 for the smash cakes. They still only get 1 layer... unless they want to pay more!

I agree with Carol Ann. I usuallt make smash cakes out of left over batter in a 4-inch single layer, decorated simply to match the cake. I dont usually charge for them if the cake order is over $50. Mostly because I use leftover batter though.

But they want 4 smash cakes in 4 different flavors? Is that different flavor icings too? Different flavored fillings? No way I would do 4 smash cakes for free, and if they want different flavors for each, you are going to have to make a batch of batter for each kind, with left overs. I would charge her $10 apiece, at least. Even if you use cake mixes, thats $1 for the mix, plus 3 eggs, plus the milk, plus the oil, plus the pan lube, etc. I could go on and on.

To make a 4-inch cake doesnt cost less, just because they are getting less product, doesnt mean you spend less to make it.