Why are general aviation planes so expensive? My question simply is why are GA planes so expensive?



I can understand that an airliner is an incredible money sink simple because of the size, complexity, development cost and especially the certification process.

But I do not completely understand why GA planes are so expensive, 
because:


most GA planes are not luxurious 
they have considerably less features than modern cars
there shouldn't be a lot of development cost to cover because a lot of GA plane designs are quite old and "simple"


Possible reasons I can see are:


there are no cheap regulations/certifications for privately used
planes
the overall production amount is not enough to profit from the
economy of scale




Further clarification:


I'm interested in the reasons for purchase and operating cost.
To narrow down the scope of the question I mean planes in the class
of a Cessna 172
location of interest is Europe/Germany but a more general answer would be appreciated to give the question/answer more weight
Simple: short production numbers. If light airplanes were as common as motor cars, manufactured by the millions, they would be much cheaper...