What’s hard to measure is the financial impact if your software engineers ship a bug. Would you lose one customer? Would you lose a transaction? What would be the cost? You won’t know until it happens. The cost will depend on your business model and how harmful was the bug.
It will happen, trust me. In all my years as a software developer, I haven’t met anyone that could code bugs-free. Creating bugs is part of the coding process, and is how we become better at coding.
Since you won’t know the real cost beforehand, you should consider hiring a QA Engineers as buying insurance. You won’t drive a BMW without having insurance, the same way as you shouldn’t build a product without having a QA Engineer.