Not that I disagree, wigglyamp, but being a hardware/software designer of some 30 years (and probably capable of developing a Garmin 530 if I had enough years to do it) let me point out a few things:
1. I would not use crimp connectors; I would solder if at all possible, and heatshrink sleeve on top. And use sealed connectors - these can be labour intensive to terminate so avionics people avoid them but they are the only way to get long term reliability. Crimps are responsible for an awful lot of intermittent problems - usually many years later. Even the better car alarm fitters now solder - crimps give too many comebacks.
2. The correct grade cables are easy to get but not in short lengths; if one needs a few yards the best way is to buy it from ..... the friendly avionics shop
3. The test equipment can be rented and many avionics shops do that
4. It isn't always calibrated. I've seen some with very old 'calibrated' stickers. Another reason for borrowing/renting - it increases the utilisation.
5. A lot of the EASA (or FAA for that matter) applications can be quickly generated or adapted from templates provided in the back of the installation manuals. In fact there are avionics shops who openly sell such "paper packages" to other avionics shops. This is a good thing of course because it spreads the cost, but don't let the avionics shop tell you they had to generate all 25 pages each time.
GA is always going to be full of moaners but there would be a lot fewer people moaning about avionics shops if they all did consistently good work, to spec and on time. Currently, even getting a written quotation is like pulling teeth with a wire stripper - the stampede for Mode S has made even replying to emails a waste of time for many. Often, some function does not work (due to subtle equipment inter-operability) but the customer is left with abandoning it as a fait accompli. Finally, I wonder what they do for 3 years studying; you get a full time electronics degree in 3 years.
Avionics is never going to make one rich because most people want something for nothing, but one should not suggest it is something extra complicated. It is just another specialist wiring + metal hacking discipline.