Some possible designs
If you select a square as a shape of the garden then the dimension are 6 by 6.
The volume of the dirt would be \(18 m^3\). You can determine the cost using what you researched.
The perimeter of the garden is 24 m. The area of the wall is \(12m^2\).
If you selected a circle, the volume of dirt is the same approximately.
The radius is: \(\sqrt{\dfrac{36}{π}}\) or approximately \(3.38 m\)
The perimeter of the circular garden is approximately \(21.24 m\), so the area of the wall is approximately \(10.62 m^2\).
There could be a design a regular hexagon or an L-shaped with say one piece of has an area of 24 and the other part an area of 12.