If it's optional, they need to know to make a decision to include or exclude it.
If it's not optional for the project, but the project itself is optional, they need to know to make a decision to kill or keep the project.
If it's not optional to the project , and the project is not optional to the business, they need to know to decide whether they keep or fold the business.
In any case, if they might choose to keep the business, project, and feature, they need to know because they have to budget for the cost if they do so.
So, essentially, business always needs to know the cost.
If it's optional, they need to know to make a decision to include or exclude it.
If it's not optional for the project, but the project itself is optional, they need to know to make a decision to kill or keep the project.
If it's not optional to the project , and the project is not optional to the business, they need to know to decide whether they keep or fold the business.
In any case, if they might choose to keep the business, project, and feature, they need to know because they have to budget for the cost if they do so.
So, essentially, business always needs to know the cost.