I manufacture plastic parts overseas and the reason all the Mopar stuff is more expensive is because of the cost of the molds. After the mold is made the cost to make a panel, be it a Ford, GM or Mopar panel is basically the same, it is just plastic which is relatively cheap. Part of the issue for a rear set of panel you need 2 molds, one for the right side and one for the left side, and the cost to create each mold is into the tens of thousands of dollars. I usually deal in small molds, like a mold for a distributor cap, a mold that size can be $3,000-$7,000. A huge mold like the door panel one will be into the tens of thousands. Now, the issue is how many of these will you sell immediately and in the short term? You do not want to invest, lets say $40-$70,000 in molds and need to wait decades to get your money back, you need to recoup your investment in basically the first couple of shipments, that should last you 6-18 months. So, you order 200 pairs of panels for a Mopar (400 total), that cost $100 each landed at your warehouse. So, you have $40,000 in those parts, plus lets say another $60,000 in mold costs; $100,000 cost. Divide $100,000 cost between the 400 pieces and they cost you $250 each, $500 for the pair, retail $1000 for the pair. Now, take this same example on a vehicle that is a lot more popular, like a Mustang and now you can order 2000 pairs instead of 200, they still cost $100 landed to your warehouse, and you still have $60,000 in mold costs. So, now you have $460,000 total landed costs for the 4000 pieces or $115 per each or $230 a pair, retail $460 for the pair, and this is why the Ford/GM stuff costs half of what the Mopar stuff costs. It is all in the mold costs and how that cost get ameliorated because the volume of parts you order.
If you were to look at this as a very long term run of parts you could eventually lower the selling price of the Mopar parts as your mold costs would have been paid a long time ago, but in reality no one does this. Once a price is established it will rarely go down, unless there is an outside influence, like someone else making the same part and lowering the price. Since a lot of these parts have so little volume, it is quite rare for a new player to jump into the market and invest all the money needed to do it, unless the available product has quality/fitment issues, that they think they can correct with a new product, and thus corner the market. But usually when a new player comes into the market with a better product, the price usually ends up even higher than what the original one sold for, and you eventually end up with a good, better, best part with corresponding pricing.
