Airplanes are expensive, both to buy and to maintain. A simple friggin stobe tube cost $96. Aircraft parts cost several times as much as car parts but really aren't any better - they just have to comply with very expensive certification requirements. A lot of people neglect auto maintenance. In a car, that can result in inconvience while you wait for a tow truck. In a plane, that can kill you.

If they could get the production rate high enough, then the parts cost can drop. Back in the 1970s, the General Aviation companies sold over 10,000 planes a year. Today, the total is about 1/10th that much. Back in 1975, you could buy a new Cessna 172 for under $40,000. Today, it costs closer to $200,000. Sure, inflation accounts for a lot of the difference, but lower production quantities drives up the per unit costs substancially.

NASA has worked with several avionics companies to develop a fantastic technology called "Highway in the Sky." It makes flying, even in IFR conditions, literally child's play. Unfortunately, the only certified installation that I know the price of costs $60,000. You'd have to get the cost down to under $10,000 to create a $100,000 plane. A 200 HP Lycoming engine costs well over $20,000. That price will have to come down as well, or they'd need to get better engines like the DeltaHawk into large scale production.

The big bugaboo is certification costs. The simple 2 seat Liberty XL-2 cost something like $20 million to get certified. A flying car would cost a lot more. You'd have to be able to sell a lot of the things to recoup the investment and certification costs.