Hey eusebius, welcome to the forum! I frequently check the Whisky Exchange for prices, as they are reasonable, and have lots of stuff, hard to find etc. And a quick check at other top shops, Royal Mile Whisky, Loch Fyne Whisky Shop, Oddbins, etc. And I'll also check other shops around the world to compare, and various ebay sites too.
I'm surprised to read ebay logs past sales?? They seem to take it down quick after bidding is over. I'll have to check closer, as I do watch to see how certain bottlings do. Basically though, it ends up being what you, or someone else is willing to spend, or get to part with a bottle.