I went through this in '85 or '86 when I first got my car. The parts THEN to convert the filler from under the deck to through the quarter roof were something like $480 to $500, THEN.
The door for $181 ain't a bad price. Plus this isn't some kind of a Chevy Chevette. These part sources are the same folks that make them for Ferrari, Lamborghini, Maserati, etc. Detomaso's aren't some kind of jabonga Italian go-cart made out of linguini.
They cost, what they cost. Like BW says, you could make them yourself.
The "quick fill", no stock filler is a great option to go non stock but if you want factory parts, no one is going to sell you parts at a sub market level and have to buy them back for ten times that in the future.
Welcome to Italian super cars.