They aren't technically wrong. To create magic items Doug used the in built magic item forge to create them (where that was possible); and he wrote some code that would calculate the cost of the item (since previously it didn't). So the code takes the cost of the base item and then adds in the cost of the magic part, which is determined by it's rarity. If the base item cost isn't expressed in gold pieces the cost is worked out by converting the rarity cost to the currency of the base item. In the example that you mentioned the cost of a single needle is 2cp and since the cost of an uncommon item is 400gp, the latter is converted to cp (40,000) and added to the base cost resulting in a total of 40,0002cp. Now when I Q&A'd the book I actually converted a lot of prices into gp (since I wasn't aware of what was happening at the time), and I obviously missed a few. But the prices are technically correct.

