Although Ellotris answer was correct at the time, this has now been updated.
It's no longer 100,000,000 gold, the new max bid for houses is 10,000,000,000 gold!
So players better bank up if they really want a house >_>
With the implementation of new rents for all houses and guildhalls, we have also increased the maximum bid amount to 10,000,000,000 gold
You can read it on the official site HERE