Around here, if it is in decent condition, a matching number Luger will generally sell for for around $1000-$1200 +or-. Last year I picked up a 1937 S/42 in about the same 85% condition as the one you describe, all matching including the magazine along with it's original WWII holster and the extra (non matching) magazine and loading tool for $1250. I also picked up a matching number, except for the magazine, 1920 dated Police Luger marked to the Berlin PD for $750. I tend to shop for deals, so I usually get martial guns for a bit less than the going rate, but contrary to the prices that high end marketers like Simpsons ask for their guns, garden variety Lugers are not made of gold.
A lot depends on just what sort of WWII Luger it is. The early pre war letter series guns will bring more as will a Krieghoff. There are a myriad of subtle differences that can affect the value. Any original accessories that come with it will of course make it more valuable.
An 85% gun is not in good enough condition to attract the more well heeled collectors and generally speaking will not warrant a high price unless it is something rare like a Navy marked Krieghoff.
Of course the price point depends on if you are buying or selling. If you are selling, how you market it will largely determine how much you will realize off of the sale and if you are buying, it will depend on just how proud of it the seller is.