85% condition is pretty uncommon for a K-38.... usually these were target guns and didn't see too much time in the field.... unless of course this was a Police Gun (many PD's issued K-38's.... some even 6" guns....). But if this was a cop's gun the Single Action feature doesn't make much sense.... during the early-to-mid 1980's Smith & Wesson sold a Drop In Single Action Kit and this may be what was inserting into your gun.... maybe....
All of this is the long way round the barn and you didn't ask for a history lesson... 3-screw K-38's aren't real hot sellers anyway, and anyone who is going to buy this specialized gun would really want to have it.... I don't think that the SA feature does alot for the gun, unless you have documentation that the gun actually shipped that way.... (original box, or factory letter).
So... I would say that a well used, but otherwise all correct K-38 with a Single Action Only Trigger would probably only fetch around $325-350 around here.... asking prices may be higher, but I don't think it would actually sell for more than that.... add about $50 for the original box if in good condition and with tools & cleaning kit.
To give you a reference point, I sold a 14-3 SA that I bought new in 1979 (box was marked "SA") for $525 to a forum member about a year ago. I think I shot 200-300 rounds thru it over its lifetime. It was in 99.5% condition. $350 max sounds about right for what you describe. Of course, a picture would let us determine more accurately, due to some folk's "85%" might be different than others.
Can't add much for pricing, but I will add for the money you'l likley get one ot the most accurate guns you'll ever own, at less than any other will run. And if you reload or can find a good reload supplier (HSM 148 lwc comes to mind), it will be one of the cheapest centerfires you'll own too. And at 85%, you'll never be afraid to take it to the range. In my wife's words "an easy keeper".