It is about the money, and you are misinformed. Tooling costs money. It costs the same money to make the tooling to stamp a hundred fenders or a thousand. So, if you only make a hundred the fenders cost a lot more if you're planning on making a profit. Which is fine if you're making fenders for a car that's worth a lot.
Mustangs win this battle by numbers. They can sell a ton of parts because there's a ton of cars and they're popular. Shake a tree and Mustang parts fall out, those are the real "cheap bastards". Their cars are cheap, their parts are cheap, and they're everywhere. You can guarantee a return on your tooling or development costs because you can sell large quantities at a small margin, not because Mustang guys would spend a lot of money on them (they wouldn't! I know mustang guys, and they're plenty cheap!). Your '32 Cabriolet is another perfect example. They didn't make many at all, but everyone wants one because they're the quintessential hot rod, a historical icon. So, thousands and thousands of reproduction body's get made because only the really rich jerks can afford originals. But there's more than enough demand to still sell large quantities of re-pops, because everyone wants one. Which is why there are more aftermarket '32's now than there ever were originals, and why they're not horribly outlandish to buy a complete re-popped body for. Expensive yes, but not considering you're basically buying a whole car. And when you're done you can still get a really good value for the car even though it's not an original.
And that's where the value of the car is important too. Wanna know what mopars actually have all their body parts re-popped now, just like the Ford and Chevy's? E-bodies. You can buy any body panel you need, they make them all. There are even some full reproduction bodies out there now and more in the works. Why? Not production numbers that's for sure, they had the smallest production numbers of all the mopars, just look at my last post. But they're worth more now, so it's worth it to fix them, even if you have to pay a ton for the parts. And you do have to pay a TON for E-body parts, believe me. But those jerks at Barrett Jackson will pay millions for a Hemi-Cuda, so everyone else will pay the high costs for E-body parts thinking they'll get a decent return. And for the most part they will, because the cars are worth enough to justify big restorations.
Just look at what you can buy for A-body's, it tells the whole story and it's not cheap owners. You can buy re-popped fenders, quarters, floor pans, etc. But you can't buy frame rails, they aren't reproduced yet. Why? Because an A-body with rusty frame rails is a parts car still. It would cost more money to fix than it would be worth, even if the parts were cheap (which they wouldn't be, not mustang cheap anyway). You can still buy mostly complete A-bodies that need work for under a grand. You can't buy the metal to fix them for that. The value of the car just isn't there. I bought my '74 Duster for $2k. No rust, original paint, 91k original miles, I almost managed to drive it home even. You can't make money selling frame rails for that car, because if it has a bad frame you just part it and buy another one for $2k that has a good frame, which is far less than the repairs would cost. That's just the market, they aren't big dollar cars. And while most hobbyists know they won't get all their money back, they also don't want to completely lose their butt by spending two or three times what a car is worth to fix it.
A-bodies just have it all wrong for numbers and demand. They made a bunch of them, so they aren't worth a ton. But they didn't make so many that everyone has one. And they aren't so popular that everyone wants one. The fact that the yards are drying up a little is a good thing, it shows that they're finally getting to be worth enough that folks just don't flat out scrap them. They're still not getting big money though, so the parts availability isn't going to take off anytime soon. It'll get better, but not rapidly. Because the money isn't there, it still would cost too much money to develop the tooling for the numbers that would be sold at any price. And if original parts are still out there for less, the aftermarket won't be there. Only an idiot would spend big dollars on reproduction parts when you can still get originals for less money that are in good shape.
