ᐅ A retaining cushion on the valley side is required according to the soil report.

My soil surveys state: "A valley-side cushion protection is required."

What does this mean? Do I need to build a retaining wall or something else? What additional costs can be expected from this?

I would ask the inspector what they mean by that. It could also be, for example, a fleece or fabric that holds the cushioning together.

No one can tell you anything about the costs since we have no information about what you are building (garage, high-rise building...).

"My car dealership says a warning light is on, how much will it cost?" – is roughly the same category of question :p