Let me preface this by saying I could never ever buy something like this...
But, I fully understand where the cost comes from. Something like this is not a common size. I don't see dimensions anywhere, but I would guess it's probably at least 18"x18" square, by 24" long, so that's major big bucks. Oak instead of a softwood like Pine or Douglas Fir commands a premium. This is reclaimed old growth wood, which is a higher price as well. Add in the labor cost of resawing it square, and sanding. And then you need to factor in that wholesale price from the manufacturer is typically doubled by the store for their selling price (relatively standard).
So, I certainly see where the price is coming from. But I wouldn't spend that money on it myself. And I can't see anyone paying for this.
In regards to the $6k cedar bed another poster talked about, that's semi-reasonable, but a bit more outlandish on the price I think.