Whitehatter
That may well be the case for turbines but when was the last time you looked at an aviation piston engine?
To say road vehicle engines are not built to the same exacting tolerances and specifications is completely back to front. Road vehicle engines are far superior to absolutely anything available in the aviation market.
The majority of piston engines available today (lycoming and Tcm) were designed in the 1930's and 1940's and have not really progressed very far from their original design, particulary regarding metalurgy, fuel systems and ignition systems.
So in response to the original question, the reason aircraft / components cost so much is the red tape / certification / testing that they have to go through before being allowed to commit aviation.
Hope this helps
JB