The reason it is so expensive is because it is a very rare (limited) material, real Fordite came when the renovated the paint bays at Ford in Detroit about ten years back. All of the Fordite that was scraped off the bays is all that there is in the universe. So, it became collectible. But, as prices rose and people started seeing green, they started stripping body shop paint bays across the country. But, the real car enthusiast that know to check Pantone color charts for Ford specific colors, know that there is a real difference.
True Fordite is sold by only one guy in Detroit, which can sell for up to $100 a gram. It is light material though, so it still bring a cabochon in for less than most gemstones. Anything you see on ebay or elsewhere is worthless paint bay scrapings. But, most of these sellers are lacking scruples and calling it Fordite anyways. But, good luck to them. Buyers are suckers for paying anything over a dollar for one of those. I can get you a box full of normal paint bay scrapings for free. Only a true car enthusiast would pay top dollar prices and they know to check the Pantones.
Sure, someone could just paint a rock. But, would you want to wear a painted rock? Jewelry usually has meaning for the person wearing it. Either to enhance their appearance, show off their wealth, signify that they "belong" to some group, or to display something intrinsic or inherent about themselves. I have set everything from a chip of someone's grandma's fine china in 18k that has a special significance for that person, to obscenely huge rubies into ugly cheap 10k settings. If someone is willing to pay me to make it, I will make it. It just depends on how much it is worth to you.