Right - if your engineering organization ships designs that are bad economically, the solution is to introduce a culture of predicting costs before committing to a design, and processes to help enforce that culture.

Add "expected budget, double-checked by at least one other principal engineer" to the project checklist.

Have the person most responsive for the $8m "mistake" be the person to drive that cultural change, since they now have the most credibility for why it's a useful step!