You practically have to have every bolt be traceable back to the ore the iron for the steel was smelted from (this is only a slight exaggeration). That puts a huge additional cost on every unit shipped. Software is not subject to that. You can certify it once and amortize the cost over a million copies sold. The cost of testing is also much cheaper.
It isn't just the cost of testing. The barrier to entry, no matter how modest, is not what increases the price so much. It's the pricing power acquired from the lack of competition, as the barrier prevents new entrants.
If three brands of GPS mapping device dominate the consumer market, but only one of them bothers to certify for aircraft, the exact same hardware in an aircraft dash-mount will cost many times the amount for a consumer handheld. Costs only determine prices for commodity suppliers. Everyone else charges what the market will bear.
