They're large or larger than a double decker bus. They're released well after first stage separation so they need to be as light as possible. Stupidly light. Despite being very large and very light, they need to be very resistant to vibration through hypersonic speeds and go through a huge range of temperatures. They need to separate reliably and equalize pressure during ascent. They look like a mere shell, but they're actually advanced composite structures with a fair amount of tech involved.
I don't think it's fair to count that as part of the 6M cost that you are "saving", if you just did the typical thing and threw them away you wouldn't need parachutes.
The reason for reuse isn't really the cost (though that's certainly part of it). The main driver is that they are going to become the production bottleneck if Falcon 9 reaches the flight cadence they are shooting for. The fairings are a big, unwieldy shape, and they take up a lot of room during the manufacturing process, so they can only make one at a time.
If you saw $6 million, falling from the sky, would you try to catch it? Each fairing is around $3 million, so they're trying to catch it. That gives them $6 million more per-customer they can discount the price should they need to get really competitive. It is really kind of insane that no one has tried this before.
I did look at the past few interviews I've watched, but can't easily find it and google is failing me. It was one of the long hour+ interviews, I do believe.
I think they play a pretty important role in protecting the payload from atmospheric drag among other things, and it needs to be pretty strong / reliable. That must add to the cost. Don't know what materials are used so can't say about material costs.