In v5 it is upgrade. You get bigger house, you pay more. Here you buy the same house 1, 2, 3.... they all the same, so the price must be the same.
When you go to the store and buy milk, does the first bottle cost the same as the second bottle ?
When I buy milk, the first bottle costs more than the second bottle. The price for buying 2 bottles at one time is less. But if you buy one bottle, and then get in line a second time to buy the second bottle, then the price is the same.
A lot of the logic in this game needs rethinking. Many things require that you pay exponentially more for each level, but each additional level gives you the same thing even though you paid so much more. It's called the law of diminishing returns. While it might make sense in some situations, I don't agree that it should be part of this game.