This paper presents the findings of an empirical analysis that tested several theories about the relationship between software quality, expressed in terms of defect density, and cost estimation drivers (cost factors). The study employs three cost factors: work effort, project size (measured in function points), and average assigned work per team member. also, in the paper we supported the empirical study with relationship between project size and defect. The study makes use of the ISBSG repository data collection update 12. ISBSG dataset of 6006 completed projects. Regression Analysis and stepwise Analysis of Variance (ANOVA) are used for statistical analyses. Our research shows that project size and work effort have a significant negative impact on defect density. In addition, results the relationship between project size and defect indicate a weak correlation between size and defects. This result corroborates some of the recently published empirical results. On the other hand, the average amount of work assigned to each team member has a significant positive effect on defect density. These results imply that when assigning resources to a project, these quality factors should be taken into account in order to reduce defect density and, as a result, the product's maintenance cost.