There are plenty of industrial rated ICs with extended temp support and such ... The main CPU manufacturers have long-term supported chips, including Intel.
Some technologies like CANBus actually matured in larger markets (i.e. automotive and the liked) so the prices and economies of scale are already there.
There's no doubt design can be very expensive, but you have to separate the certified and non-certified world. Having seen what it takes to create a certified products, it's absolutely mind-boggling. I understand why a GPS costs the price of a small car, and there's little to no way around it. Part of the problem is that entire systems, not just components, have to be certified together (for now anyways).
With experimental avionics, this is not required, so our market is much more primed for the use of things like open interfaces, protocols, etc. Create a common CANBus-based protocol (full stack, not just the bottom 2 OSI layers!), create some test tools and such, develop against the standard, certify against the standard, and voila.
Give the DIY nature of home builders, you'd think you could get a community growing around this and kit-build avionics