Vague pondering

If you were asked to try to demonstrate your value to your employer in terms of dollars of revenue generated or costs saved, how would you go about doing this? (I am not being asked to do this personally, but in a way our department is, and I find it an interesting exercise to try to personalize this to my own work.)

This is particularly interesting to try to measure in the case of tech writing, because a lot of our value comes from things *not* happening: customers don't get confused, don't waste time trying to complete their tasks, don't phone tech support and tie up a rep's time, don't get frustrated and decide that they're going to buy somebody else's software.

My father is clearly convinced that I contribute to my employer's revenue, because he sent me a congratulatory email when we posted our quarterly results yesterday, telling me to keep up the good work. Perhaps I should pose this question to him...