What is AC?

I‘d like to start using Cucumber in the context of domain driven design. My problem is that the initial design phase of projects never leaves space for this budget-wise. While I learned in engineering school that 10% of the project budget is to be allocated for an architectural prototype, the situation on our agency projects is completely different. It starts from selling the shiny thing and then underpinning the business logic in a move-fast-break-things manner. We are at least disciplined enough to have 100% unit-test coverage. But how do I make a proper argument for long-living domain logic tests which are readable for the business side and serve as a discussion base for business model design changes?