Why are proportioning valves so expensive?

[davepl]

I was sand-blasting my disc-drum proportioning valve when, to my shock and dismay, I reazlied I hadn't plugged one opening. Figuring I now have abrasive in it, I need to rebuild or replace it.

The problem is that I can't find anyone who rebuilds them and the replacements are close to $200.

Presuming I want to stay stock appearing (no billet aftermarket units for me) and don't want to spend $200 on a valve, do I have any options? Or am I hooped?

I could throw a used one in if I could find one, but it's sort of dodgy to skimp on a safety item in an otherwise complete restoration...