A defective planchet is a planchet with any number of problems and these problems occur before the strike and usually happen during the planchet manufacturing process.
Values on defective planchets depends on how dramatic and severe the defect, a cent with a slight defect can be worth over $30 but a dramatic example can sell for over a $1,000. However values in the $700 and up range are more rare, most values tend to hang around the $200 range since most defective planchets are not extremely dramatic.
On a further note; don’t make the mistake of thinking every damaged coin is a defective planchet because there’s damaged coins out there that can fool a novice. So if you think you have an error then let an expert examine your coin. If you can take clear and large images you can send them to an expert and they can usually help based on the images or advise you further.
Image courtesy of Heritage Auctions