Hi Spit, A good mate of mine is an expert in this area, He has always told me that if the signature is from a Fighter/ Bomber crew member and it is between 1939 - 1945 the value is obviously more due to the war.
I was looking at an old logbook 2 weeks ago, And the Fighter pilot had flown out in Rhodesia on Hurricanes, Thunderbolts etc.
He was sadly stamped in red as killed in action.
The year was I think 1946, I vaguely skiped through the book as time permitted.
My mate said it was probably not worth as much as it was post 1945.
Any 'original' signature by the dambusters crew - Gibson, Cheshire etc is worth £1,800 + that's just for the signature.
Obviously if the signature is titled flew - Hurricane, Spitfire, Lancaster the value increases dramatically.
There are a few 'fakes' on the market.
You could contact the IWM Duxford or Hendon for specialist help.