So from the ptr housing test you have an initial fee for the plot. On the test it was like 1k which is probably subject to change. I think you also have to pay to move a plot. But the house came with the plot and we could decorate it with towers and extra windows for no extra cost.
There is a renown track to level up to earn more room budget so you can place more items and rooms as you level up. So your major purchase will be the initial plot, and then whatever decor you need to buy. The vendors in the housing district were just using gold but stuff obtained out in the world will likely have additional expansion currency to farm to buy it as well.