A valley-side cushion securing is required according to the soil survey.

In my soil reports it says: "A valley-side cushion securing is required."

What does that mean? Do you have to build a retaining wall or something else? What additional costs arise from this??

I would ask the expert what he means by that. It could also be, for example, a fleece/textile that holds the cushion together.

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

"My dealership says a warning light is on, how much does that cost?" - is roughly the same category of question :p