$41K - thats impressive!
I'm kind of assuming that its not a door shell for a brand new current model C172, and that a 2006 C172 has a different door to that of a (e.g) 1973 C172.
So its fair to assume that there are no door shells in stock, and that someone, somewhere has the original tooling to make a door outer and and a door inner panel.
Whoever has the tooling will have to fit the tools to a press, stamp out the parts, roll the edges together and prime them.
I'm still assuming that these doors are un-trimmed, no glazing, handles, interior trim etc.
But its still an eye-watering wodge of dosh - £23,587, I do hope that includes VAT.
For that sort of money, you could actual buy a complete C172.