Is this Greeves yours, I sold my challenger for $6000. but, My Greeves was totally original, as for yours the guards are not, but that`s here or there. The pipe looks wrong, I could buy back my Mk5 challenger for about $4500. that is some indication.
That bike in the photo, Looks to be a mark 1 or 2 Greeves between $4500 to $5500 tops, if its a good running bike. if not reduce the bike.