Depends on the steerer length, the longer ones can be cut down so will fit a wider range of frames. Paint scheme and colour will play a role too, giving the number of frames it can reasonably be matched with without a respray.
That said, I'd say £75 (short steerer, tatty, odd colour) up to £150 (long steerer, good condition, usable colour like plain black).
I'd certainly have paid the upper end of that when I was looking for one - in the end I sold the frame because I couldn't find a fork...