It is generally believed that the late a software bug is detected and fixed, the more is its bug fixing cost. Bug fixing cost is always a major issue in software development process as it can cause schedule slips, overrun of budget and finally an unreliable delivered software product. This work proposes a simple yet effective Cost Model to estimate bug fixing cost and is applicable in every software development phase irrespective of where the bug is found. This work also shows that very costly bugs were caused due to specification deficiencies discovered only in the function testing phase. As such it confirms that in order to achieve lower cost and a high quality system, Quality Assurance must not be isolated to a single phase in the development process, but rather, incorporated into each development phase. This book serves as a guideline for anyone involved in software development who wants to minimize the bug fixing cost and wants to uphold the project schedule.