There's a little problem with the assumption of economies of scale in this particular case; it's not a high volume product by nature and it never will be.
1) The sheer cost of parts for the RD motor (I can't say for Dinan since I haven't seen manufacturer receipts, but I'm positive it's high as well) is higher than the MSRP of supercharger kits currently on the market. It doesn't use a single off the shelf part. The labour cost of actually having an extremely skilled technician dismantle and rebuild the entire motor is fixed as well, plus the cost of having someone equally skilled increase the bore size of the block (your average shop cannot do it with enough precision). In addition, the cost of the exhaust, headers, etc... which are all hand made and included in the price in RD's case.
2) The companies that manufacture the pistons, crank, rods, etc... for RD are all of the highest quality and world renowned in both the OEM and racing worlds. Even if RD built fifty motors, that pales in comparison to BMW building tens of thousands, or a race team building hundreds of motors and the associated re-builds.
3) The idea of bulk purchasing to reduce costs only applies if all parts are ordered at one time. Does it make sense for either Dinan or RD to buy eighty pistons, ten cranks, etc... and stock them for the next ten potential customers? Absolutely not since the cost outlay would be enormous, and it's not worth having these parts sit on a shelf. Plus I don't think there would even be a substantial discount on many of the parts even by ordering enough for ten motors.
It is absolutely impossible to build a stroker motor for this car for $12k or $15k. Even if you sold 2x-3x as much there would be very little parts cost benefit to the manufacturer, and the resulting losses would be huge.
If you don't see the inherent value of the parts and labour associated with a fully rebuilt motor then most likely a supercharger will give you everything you want. Does it give more bang for your buck? Is it better value for money? Those answers depend on where your priorities lie.