AmazonUK has this Supernatural box set priced at AU$84 delivered. Cheapest that I've seen.
You have to view it via the Film & TV: Deals of the Week to get it at GBP51. If you buy it via direct product link you'll be charged GBP69.
Also it states there that, "Temporarily out of stock. Order now and we'll deliver when available. We'll e-mail you with an estimated delivery date as soon as we have more information. Your account will only be charged when we ship the item. Dispatched from and sold by Amazon."
Seems like Supernatural is being sold everywhere at lowered prices. Next blu-ray (Season 9) will be released on October 2014.
I am tempted to buy now. Argh.