Why making an airplane engine is so expensive ???

To answer this first we have to know that the aircraft engines run for hours at high temperatures, and they must resist it without breaking.

That's why your usually use titanium to build the engines. But the titanic is expensive. The plane in general is expensive. Very few people save that the price of the airplanes is quite similar to the one of the rocket. But the rocket are only used once, and the planes are being used. That's why it's not worth a plane ticket than a plane ticket.

For all this, and because the engine has thousands of sensors, aircraft engines are so expensive and difficult to build.

http://www.bbc.com/autos/story/20170214-how-jet-engines-are-made