There are many possible combinations of 2-layer and 3-layer cakes.
First combination
0 2-layer cakes and 10 3-layer cakes, total cost = $30
3 2-layer cakes and 8 3-layer cakes, total cost = $27
6 2-layer cakes and 6 3-layer cakes, total cost = $24
9 2-layer cakes and 4 3-layer cakes, total cost = $21
12 2-layer cakes and 2 3-layer cakes, total cost = $18
15 2-layer cakes and 0 3-layer cakes, total cost = $15