Adjust Design Length

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However I would suggest the both longer and shorter should cost more! Overall cost that is.

I'm not sure that would make sense. Why should it cost more if it takes longer? In absolute terms it should cost less.

If there is an optimal point and things grow more expensive in both direction, a company would just choose to do it in the suggested time and save the $. If its a sliding scale from amazingly expensive and quicker down to just moderately expensive and slower. Then the player has to weigh up how quickly they are going to need something and how much they are willing to pay.