If it's a UK copyright, 70 years after death of author, so as VM died in 1959 you've got a few years left to go. The price you have been quoted lies in that edition's rarity I should imagine.
As it's still in copyright everything else is hypothetical.
See P-01: UK Copyright Law fact sheet
If it has US copyright have a look at
Copyright Term and the Public Domain in the United States
According to Wiki ' Towards the end of his life he was naturalised as a U.S. citizen.' so it could come under US law.