You'll want to price out handling when the domain expert's code breaks, and the exponentially increasing inference costs as the code grows.
I’ve heard this a thousand times: You know that small stupid tool you wrote last week? Everyone is using it and you must adapt it to all our services (let’s say 100) by the end of the week. Ask if you need help, we’ll ignore you because we don’t have any specs.
Also as OP said: "a domain expert who understands process" What happens when that expert retires?