Depends if your buying from a good dealer or from Atlantic Motorcycles.
Those thieving gits have taken the mick out of classic bike prices. They even advertised a BSA B31 for £4000. They are rip off merchants that bump up the prices to sell to the states.
I reckon on £18-25k for a Black Shadow depending on cond.
My dad gave away a Series B rapide to a friend of his in 1956, he's been kicking himself ever since.