While it's true that Quality Assurance is a function of the software development process, it is also a business tool that can have a dramatic effect on the profitability of your company.
In terms of ROI, there is a "sliding scale". The longer it takes to find software bugs, the more expensive the process becomes. In Quality Assurance jargon, this is called the cost of quality or "COQ". There are 3 COQ categories:
Prevention: As the name implies, this is an effort to prevent errors from occurring in the first place. Your investment will come in the form of establishing QA methods and procedures and training. This is money well spent. I think of it as the QA version of "value investing" in stocks. These costs are incurred prior to product design.
Appraisal: This is the development phase of the project where costs come in the form of product reviews, including requirements vs. product features, code inspections, and testing. This is when things get more expensive. Errors found in this phase must be corrected before delivery and involve re-work in the form of revised software and additional testing.
Failure: This is the most expensive category. Defects that were introduced due to a lack of (or ineffective) procedures in the prevention and appraisal steps are discovered here. The costs here can be astronomical. Defects may be found prior to delivery, thereby delaying delivery, or worse yet, found by customers.
The expenditures in each phase above plus the initial cost of production, comprise the total needed to build a product. The total cost can be greatly reduced by conforming to QA standards within each category.