They quoted me $400 in March 2021 + $100 to install it. If you forgo installation, there is a $50 shipping charge. I tried to clarify I didn't need a door, just the roller and they confirmed that the quote wasn't a door, "just the part I need".
I understand the door is 25 year old and I don't expect them to have parts available for forever. Had they said it was out-of-stock and they couldn't get more I would have grunted and moved on. Insisting on $500 for a part for what was at most a $300 door was a bit infuriating.
So, I reverse engineered the end parts in CAD and made 3D prints of them. One end is pretty complicated and it printed pretty rough. I am working through refining the design to be easy to make but still fit on the door and in the track. If I come up with something that does work it should be easy to reproduce for a couple of dollars. In which case I'll share and might even be willing to ship a few at cost. I have been planning to re-use the wheels, but it looks like standard 3/4" (19mm) shower door rollers might be very easy to adapt. They are only $3 for 4 and have nice ball bearings...
I'll probably have more than $500 in time in this project by the time I'm done, but somehow I will still feel better about that