Evaluating the Performance of an IT Department?

Daniele Pagano asks: "I have just been promoted from code monkey to manager of the IT department of a construction subcontractor. As far as this industry goes, we are relatively high-tech (and getting better), but like many IT departments we are often considered a necessary money 'black hole' (i.e. they just notice the outages). So one question that arises is: how do we actually value our work? That is, how much money are we actually saving the company, and how do we demonstrate it to them? How do we value the contributions of each IT staff member (say, for a bonus or raise) in an objective, quantifiable manner? I know there is no one correct answer, and I have many ideas, but I thought we could pull our thoughts together for the betterment of small IT departments everywhere."