This thread will probably do better in the general air gun section so I will move it to there for you. Do please remember however, that buying/selling off a valuation thread will earn all involved a holiday away from the BBS (see the stickies/rules). May also be prudent to post a few links to a few photos here, too, as without photos, prices will be very vague.
FWIW here, I sold a 25th anniversary Midas edition Huntsman .177 (one of only 1000 so quite rare) with a x32 mag Tasco scope and carbon mod for £550 collected last year, so the one you are talking about will probably be worth around £400/£450 rifle-only. Hope this all (sort of) helps . Atb: G.