I think you're focusing on the wrong thing in your analysis of prices. -It's not that either version of the mag costs significantly more to make, it is the old issue of supply and demand.
-Most states do not have a limit on magazine capacity. People who have had the ten rounders are dumping them and replacing them with full capacity magazines. The ten rounders in the various supply pipelines are probably far greater in number than the demand for them in the few states that limit magazine capacity to ten rounds. That excess of ten rounders and the low interest in them has caused their prices to fall. -As anyone in retail will tell you, it's better to move an item out, even at a loss, than to clutter up valuable space and tie up capital with an item that will not sell.
I'd expect the full capacity magazines to sell briskly at $35-$40. -In all candor, after being asked for a decade to spend around $70 for pre-ban, often used specimens of these magazines, $35-$40 a mag is a good deal. -You are of course free to wait for the price to drop to $28, but it might be a very long while, if ever, before that happens.