I can at least provide a reference point for Zento house cost. I own one just outside the gate between the water and west wall. It is a great location, and I can't remember if it is 18x18 or 17x17, but it is one of the bigger sizes. I bought it four years ago for the equivalent of 4 mil gold. I say equivalent because I traded a 4.4 Greater Dragon (this is a few years before new training) which was a rare (and valuable) pet at that time. I actually regretted trading the dragon after that for a little while, though now that I have the Museum set up I'm happy. When Tyrath left that freed up several Zento spots but I don't know what the new acct owners did with them. I believe there was much trading and swapping. I traded the Tsuki Wolf House to Planters for a vet reward Brazier, and I think he had to get rid of at least one Zento house to free up an acct for it.
Edit: To be fair, I should probably mention that four years ago the shard was at a lower activity level. Demand does seem a bit higher now, though I'm not sure it is an order of magnitude higher.
Second edit: I'm not sure the majority of folks spend 75% of their time in their house. I'm generally only in there to craft or chat with a friend.