What is the expected value of owning a defibrillator?

None: I assume you're referring to an AED - automatic external defibrillator. Unless you live in a cardiac care unit, an airport, or some other large public area, you'll pay a lot of money and never use it. It won't benefit YOU since someone else has to apply it. It must be tested and certified annually (another expense). (If you're talking about an INTERNAL defibrillator, disregard all of the above)