The cost breakdown would look like:
$243 for a copper plate from Online Metals
$60 for machining from FPE
$100 at least for shipping the plate to you from Online Metals, then to FPE, then back to you.
Is a copper plate worth $400? You could take the plate we provide to a powder coater and get a copper color applied for less than half that, and it won't constantly try to oxidize on you!
About ten years ago I worked for FPE and had something setup so people could custom order Crack plates. The boss there wasn't exactly super happy about it, and nothing like this ever existed for the Mainline.