What's the right amount of test coverage? Test Coverage and Post-Verification Defects: A Multiple Case Study](https://onedrive.live.com/redir?resid=493B8BCE8FC1C3DA!26920&authkey=!AFbVeq8YcyR0jDQ&ithint=file%2cpdf) provides some insights. Interestingly, they find:

that the test effort increases exponentially with test coverage, but that the reduction in field defects increases linearly with test coverage.

In other words, it takes more and more effort to reduce field issues. Based on their study most of the benefit accrues up to around 80% code coverage.