I'd also disagree that the sphinx decals are rare. In looking for almost a year, that is by far the most common pattern I have found. And I've never seen a puzzle box sell for much over $100 and this is for one of the rare styles with ALL of the parts. More often they sell for $25-$50 range (including shipping).
But it does no harm to ask for a lot of money for something - and you never know . . .