All pretty good points. If production was up to a reasonable rate the cost would come down some. While we have a lot better machining technology today, the 250 was made using dedicated machines. That is, each operation had a dedicated machine and that reduced costs a lot. While you can do more nowadays with more capable machines, it doesn't reduce costs as much as you might think compared to production line methods.
RR right now isn't making their cost bogeys for the RR300, and is, consequently, losing money on each engine. This is because they have high costs and low productivity in a union environment and their costs are higher than if they bought all the parts and just assembled the engines. RR gets a big number for the other Model 250 engines and makes a good profit on them.
I'm sure the PT-6 could be sold for less if P&W had some competition, but they don't so the price is what the traffic will bear.
Nobody has mentioned the cost impact of product liability, which is a big deal for engines. You have to factor that in to the price too. It isn't just the cost of making and sending an engine out the door and covering the warranty costs.
Turbines can be less expensive if the engine is designed expressly for low cost production using modern production techniques. But you aren't going to ever see a 300 hp turbine with a cost similar to a 300 hp recip. The reason is that the nickel base alloys in a turbine expensive, and all of these nickel based castings for the hot section pieces are vacuum investment castings and those are expensive.
Don't try to compare model turbojets to larger multi-spool man rated turboshaft engines. Those engines are very simple machines with relatively low mechanical loadings. Consequently the performance is poor and they don't operate at higher temperatures or require high performance and costly materials. They don't have power turbine sections and gearboxes and don't have fuel controls that would ever be capable of being used on a real engine. They aren't man rated and I'm sure the makers aren't carrying the product liability insurance that would be required for a real engine. They are basically toys when compared to real engines, and you are paying the price of a toy for what you are getting.