The costs depend on your application and your application domain and might vary a lot. If you are developing software for an aircraft, the costs of a software defect might be huge, whereas if you are developing software for a pet project the possible costs of a defect might be close to zero.
Costs might consist of claims because of liability, but most of the costs are indirect and hard to measure. Think about the loss of reputation if a software bug in your system shows up in the news. Or, at a smaller scale, if your software is not maintainable, lots of extra costs are needed to add a new feature.