Manufacturing economics is something you would do well to understand. There is a big difference between a part that is generic and applies to millions of vehicles versus a part that is unique to a particular vehicle, especially mk3 supra which had pretty small production run numbers, and is shrinking steadily with every part out. The non-recurring engineering expenses for a run of custom parts gets divided by the total number of parts made, and for these turn signals that is a pretty small number, so the price is higher.
By the way, a single stock turn signal housing sells for $63.93 from the dealer, so the ebay ones are actually pretty fairly priced.