The cost of defects can be measured by the impact of the defects and when we find them. Earlier the defect is found lesser is the cost of defect. For example if error is found in the requirement specifications during requirements gathering and analysis, then it is somewhat cheap to fix it. The correction to the requirement specification can be done and then it can be re-issued.
In the same way when defect or error is found in the design during design review then the design can be corrected and it can be re-issued. But if the error is not caught in the specifications and is not found till the user acceptance then the cost to fix those errors or defects will be way too expensive.
If the error is made and the consequent defect is detected in the requirements phase then it is relatively cheap to fix it.