Diablo – How much is the Plan: Staff of Herding worth

As far as I can tell, there isn't one. I purchased an item at the auction house using Buyout so that I would know immediately when the auction ended. After a few seconds pause my buyout was accepted and the item was available in my Completed tab. I clicked to move it to my stash, and then went to the Sell tab and was able to immediately put the item back up for auction.

Note that this isn't 100% certain, but this is how it appears to work from several commodity sales and purchases in the past few days:

• The auction house automatically purchases the cheapest items, but quotes you for the current average of the items within your quantity. If cheaper items appear between your search and your purchase, you'll receive a refund equal to the gold saved by not having to purchase more expensive items.
• If the purchase fails due to another player's purchase making your current gold insufficient for the quantity of items you want, the transaction will fail and you'll receive a full Purchase Refund.