The costs of certification, development and liability insurance are adding a lot to the price.
People (me included) complain about the same thing when it comes to 'marine' parts - they're often 2 - 3 times higher in price than land-based parts (usually automotive or RV parts). There are actual differences in the items in many cases though - for example, marine engine parts have to ensure they never throw any spark, because explosive fumes in an enclosed engine compartment are a real safety concern that well ventilated automotive engines don't have. That takes extra R & D and different manufacturing processes for much smaller production runs (increasing pro-rated capital costs).
Plus, market forces seem to show that people are willing to pay the higher prices - perhaps that's in part because most of the people buying these things have more money than people buying commodity (automotive etc) parts.
What planes are you referring to? I'm not familiar with models in that price range that are certified for commercial operations.