I know that its expensive, but they don't need to recoup their initial investment all at once, spread it out over time. How many years has the rear valance panel been out and it's still $400, that's greed
No, that's economics. It's not a matter of how many years the valance panel has been out, it's a matter of how many they need to sell just to break even. And there isn't ANY company that's in business just to "break even".
On a low demand part (be honest with yourself- how many people are actually going to be buying the part? These aren't Big Macs), it could take decades to recoup costs- so just be glad someone was willing to take the leap at whatever price. If you don't like the cost, break out a hammer, a piece of sheet metal and make your own; it's what you'd be doing if nobody reproduced the part anyways.
You think you've got it bad? Buy a Studebaker.