Tester fills in bug log with test that fails and information available. If we are really lucky, they pass on the bug reports to the original programmer team. If not, multiply the following by some arbitrary multiplicative factor to deal with the fact that people not familiar with the original program are faced with fixing it. They investigate the design to see why it produced that error. They write new unit test, refactor, add code, etc until it passes, and mark the bug report corrected.
Tester picks up fixed bug list, finds the test to run, reruns the test, it passes.
Notice that there are at least three people in this story, not just 2. Call it 30 - 120 minutes Finding the same mistake was on the order of 10 times more expensive in this scenario. While the ratio could easily get larger, it won't get much smaller. But it sure does reduce the number of bugs getting to the external test department, thank goodness. Writing all of those unit tests sure does cost time and energy.