$300 a pair would be "reasonable" but not $600 for a chunk of plastic. Yep, you can find ANYTHING for a Chevy. But there should be somewhere in between as other repro lenses are out there at reasonable prices for Packards (not just the '56 Clipper that everyone uses for streetrods, but the two-years only Clippers, the '55-56 seniors), the one-year only 1958 Edsel, the one-year only '54 Hudson, Studebakers, Nashes, et. al. I'd settle for a lens w/o bottom screw holes and I'd emblem seal it on the car. no big deal.
Trim parts indeed! How about molding clips? Hundreds, maybe thousands out there but ever try to find the 28 clips that hold the '51 Frazer hardtop external rear window moldings on? The big name brands suppliers don't have them. I finally found a place up in Ohio that had some I could modify to use, but I had to cut and file all 28 of them down to fit.
And going on FOUR years now trying to find a left front vent window frame for the '51 hardtop. The inside and outside door moldings in ANY condition forget about it based on a four year search for those as well. Sad thing about the vent window frame is I'm fairly certain some exist but to date no one has come back and replied they have it or will go look for it.
I would NEVER again undertake to restore a '51 Frazer or any other rare model car based on what I've learned the hard way over the last four years. I'll even throw in the '55 Studebaker in that category. Too hard to find parts.