I meant my little "project" idea as a silly thought provoking thing.
At the basis a downrigger is a wheel, crank and an arm to hold the line away from the boat. All downriggers started as a concept. The better ones ended up getting sold and boom you have a new company. My design idea was based on how a baitcaster reel works, but bigger and could hold way more line. One person mentioned the issue with different metals and salt water, which is a valid point I hadn't thought about. But since it was mentioned I would keep that in mind if I were to build one.
In this thread we were talking about what downrigger was best and why. And what flaws we find with some downriggers. Maybe someone has worked out a way to modify their scotty or cannon to make it work better?
The "I Built it myself" factor has always been big for me and I know a lot of other people like that. And there's a whole market for that kind of thing in the car world and even jonboats are heading in that direction now.

Oh and my main question was, why are downriggers so expensive. I honestly don't know. I would assume the research that went into them and the brand name play a big part. The parts themselves are relatively cheap to obtain.
My view is from a complete newb to the boating world and in fact never used a downrigger. I got 2 the other night and have yet to try them out.
We'll see if I come back with all my fingers.