<p>Unit tests are cheaper to write and maintain, but they don't cover all scenarios. What is the right balance between them?</p>

On the app I am working on at the moment, there are probably 10:1 unit to functional tests. The unit test work simply things like retrieving entities from DB, error handling tests for db/network connectivity etc. These things run quick - minutes or less and are run by devs daily.
functional tests while fewer tend to be kitchen sink approach - can user complete order etc. tend to cover the business domain end of things are run by business analsyts and operations - sadly for us, often by hand. These things take weeks to run and usually to finalize a release cycle.