I voted $300-$400.
Thats buying with the heart all things considered (ie. build quality; smoothness of focusing; takumar heritage; optically good as lenses now)
If buying based on the mind, then it may sit $100-50 lower.
There are a lot of good 85mm out there (some non-Pentax)
The Samyang 85/1.4 is the immediate one that comes up.
Then there are the 85/1.8 by Canon and Nikon.
The EF85/1.8 for example often goes for $300 used.
Its not old style build quality, but its built well by todays standard, AF and auto-aperture, good performer throughout.
Not usable on a Pentax of course, but just a relative comparison to what is being offered elsewhere.
I recently made a choice between a EF85/1.8 or a S.Takumar 85/1.9.
The Takumar was slightly more expensive.
I bought with my heart......