if it's like this one, then it's around £220 new with reasonable match sights:
https://www.surplusstore.co.uk/air-w...b78-match.html
So £100 sounds about right bearing in mind the sights are not that cheap.
edit just found the one without opens - £175
https://www.mrserviceshazlemereltd.c...499-1177-p.asp
So £100 seems about right if it's in good nick. If it needs a reseal, knock off £20-£30
I assume it's .177 ?