The rule of thumb for swords is similar to that of scopes on rifles: that you should spend as much on the scabbard as you did on the sword, relatively speaking. If you buy from one of the top two or three scabbard makers in the world, you could easily pay $2,000 for the scabbard alone, and you'd not be getting gyped! You'd be getting the Ferrari of sword scabbards.
I can say from experience that having a crummy scabbard will drag down the look of your whole harness, and having a fantastic one will do the opposite.
Scott