Yeah, seems like a great item to bump up the price since everyone will changing them after this letter. Should never be this much. I just bought some from grove and it was 80 cents each.
I would at least make sure you get mil-spec o-rings. I just redid my brakes, and I used o-rings based on the size "equivalent mil-spec size". As soon as I had everything reassembled, and used the brakes, they were leaking from the top.
For example, Aircraft Spruce shows MS28775-010 (just a random example to show the point) as:
ID: 1/4 (.25)
OD: 3/8 (0.375)
Width: 1/16 (0.0625)
So I got the ole o-ring box out and found those sizes. Leaked.
If you look at the actual mil-spec, MS28775-010 is:
ID: 0.234 - 0.244 (always smaller inside hole)
OD: 0.368 - 0.390 (possibly bigger around, but on average the same)
Width: 0.067 - 0.073 (always taller)
When I got them and lay them side by side to the ones from the o-ring box that are supposedly the same size, you can perceive the differences in them by sight. Separately I don't know that you could ever tell. Haven't had a leak since, and as I assembled my master cylinders, these o-rings were noticeably snugger on assembly, and in operation.
I would make sure you get actual mil-spec o-rings.