VOIP, how does it save money over PBX

I get the main gist of how VOIP systems work. But, what I need help with is how it saves money over the standard PBX system currently in use and already wired into a building. You still need to buy the phone lines, and numbers from the phone company. IP phone systems aren’t cheap either, and support is about equivalent to paying for PBX support. I’m all for moving into the future and think that IP telepony systems are the way to go; they have excellent features, and with some they are getting closer to being one phone for everything in and out of the office.
How do I show my boss and up the ladder how spending 60 to 100k on this new system will be better than just updating what we have currently?