I am a mechanical engineering consultant and from my experience (which may not be worth much in this forum), if I am on a design project I must be able to design in most cases a product that will yield a MINIMUM of 50% margin. If not, I would starve to death. If you want my professional opinion there is a min. of 50% margin. After development cost are re-couped (tooling, fixtures, R&D) the margins will be much higher.