15 - 20 years ago used factory power steering sold for $500. I personally have sold a couple dozen setups over the years for $500. You'd think as time went on the price would go up due to scarcity. This has not been my experience at all. With all the options out there now, factory stuff isn't as popular as it once was.
I used to have no problem selling two or three setups a year when I had them. The current one I have has been sitting on craigslist and this website for three years with minimal to no interest.
I'd still say $500 is the target price for a used setup, but you're going to have to wait to find someone specifically looking for used OEM and not interested in a 4x4x2 or any of the new boxes that are available.