Turbine released a new test build this weekend. In the release notes, the developers talk about the updated upkeep costs for Premium Housing.
“Players may now pay gold upkeep for Premium Houses. 1g for Stately and Luxurious, 5g for Deluxe Kinship”
“Mithril Upkeep for houses has been reduced globally. The cost is now 2 Mithril Coins for individual houses and 5 Mithril Coins for kinship houses.” – source