A Cessna 172 ironically is probably worth more than an average equivalent non-Cessna plane ironically because it's so ubiquitous that parts, experience, and understanding are everywhere. It's the Ford Focus of light aircraft, beloved by those learning to fly, and actually usually holds a premium as a result. I take your point though; but the real reason everything in aviation is so expensive is that it is traceable, documented, and with known MTBFs and characteristics -- all requirements that are completely and utterly written in blood.