Are you talking the Mini House Deed or a Deed to place a castle? Either way 10 mill is steep. If it is the Mini House Deed, the hardest to get is the Thatched Roof Cottage which I just found last week for 1.5 mill. I know where there is a Keep for 5 mill, if I had more reserve coin I would do it but that is more than I want to pay at the moment. So for the mini deed I would say under 5 mill. For a reall Castle deed that is tough to say up to the buyer because youcan't X-Shard it so you are trapped with it. You do know if it is a deed to place not to keep it in your pack or in your bank box.
this is one of the worst shards to have rares on. So items are usually tough sales when they can't be xferred for. (the House Placement deeds)
if it's a Mini House Castle deed, it's worth 4-8mil. I have the 3 rare large house minis (tower keep castle) for sale for 5mil per and they haven't sold as of yet.
Teufel Hund, I'm pretty sure the rarest Mini House is the Castle, followed by the Small Stone Keep, followed by the Tower, followed by the Thatched Roof Cottage. But a Cottage for 1.5 million is still a good buy, in my opinion!