Not wanting to override what Mach 1.5 says, I have to slightly disagree with his valaution.
I would suggest a value more in the region of £250 tops. The reason i say this is due to the fact that the stock makes up a lot of what is an airmasters, this stock is more or less a write off and finding another is almost impossible.
Secondly, unless its been apart, there is no guarantee the glided internals are still in, if not, then its just a standard 77. If the inside has been misstreat as badly as the outside it will need regliding.
I just wouldn't want you getting your hopes too high.
Hope this helps.
Fozzy.