Ever replacing a xenon bulb is actually very rare. The lifespan of these bulbs is usually far greater than the lifespan of the actual car. They are also much more stable than any bulb with a filament in it. So replacement costs of HID's, unless they are physically damaged, really shouldn't even be a concern.
I actually do not have them on our Outlook and wish I did. When I got them on my other car, I swore I would never buy a car again without them...but ended up having to get the Outlook on short notice, and there were none that had the options we required that also had HID headlights. I sure wish we could have had more time to find or order one with them, though, as they are worth far more than the $500 in my opinion.