i'm sure you won't mind if the navy paid three times more for the rolls royce engine in your harrier :-)

seriously though, it's completely different. planes are very labor intensive and are only cheap because they are made in low cost labor places. try to get your hobby shop owner to make you a kit and i'm sure that would be three times more than any 70 class four stroke. even at minimum wage, you would be looking at thousand easy to build a kit to ARF level of completion. here in taiwan, where labor ain't so cheap no more, you need to pay someone $60 dollars to put a basic ARF together!!! as jeepindog said, you should just be happy that you can get them for cheaper.

engines, on the other hand, are capital intensive. So it doens't matter as much where it's built and that's why Magnum engines aren't so much different in price than O.S. engines.

lastly, think about what it would cost you to build either. i'm sure with a little arts and crafts, we can all build a plane from scratch. try building an engine from scratch.

another analogy is the computer.... you can get a case for less than a $100 but all in that puppies gonna cost you $1000+. ever try making a CPU? lol