Try pricing automotive windows. When I bought my Interstate, some vile miscreant broke out the passenger-side front window that same night and stole the Kenwood head unit out of the dashboard. (Side note, even before I bought the Interstate, I made an appointment to get an alarm installed; the thief beat me to it).
The new window alone was $250, which coincidentally was the deductable on my insurance. This for a stock Sprinter van window, not a custom cut-to-fit window. Your $230 price tag for your rear trailer window is entirely reasonable.
Engineering: Finding complex solutions to simple problems you didn't even know you had.