1. Benefit. Give FC a clear benefit when they own a FC house.
For example, 2% main attribut increase in a hour after stay in house over 5 mins
Or some benefit for crafting grinding. The bigger the house more player can get bonus from house.
Without a clear benefit, none will to buy a house with such high price even they can offer even higher price. It is not real world which we have to get a house for living.

2. Resell value. House can be resell when they got more Gil to offer even larger house. So FC still will to buy a small house as soon as they can offer it.

3. Give some house for auction every week. Just like foreclosure house. Let marketing decide what's it worth instead of a game director's decision.