True enough, but then by the same logic, considering how easily one can make money, why can't the Barovians afford these homes? That aspect of it doesn't really reflect the setting, but it is what it is. Given that this is NWN and not PnP DnD, there are limitations. I understand what you're saying... I thoroughly enjoy the server and I just figured I'd mention a few things I thought of that could add to the flavor and RP in some way.
Its really only easy for those who take the time to dungeon consistently. As Bato said, there would only be a small limit of them, just like the Governor's Hotel in the port. The Penthouse has been rented out indefinitely for several months now (5000 a week, mind you), and I've heard a handful of people always 'waiting for those suites to become avaliable.' If I were to toss some numbers around..
500/Week (Same as the Hotel) Is probably affordable by most, A barovian PC could probably pull the funds together before each deadline. Someone who makes an effort to dungeon often, probably will never have to worry about losing it as long as their active. Any character with a small fortune can own the house indefinitely. Its only 26,000gp to own the house for the entire RL year.
1500/Week (Same as Guest Suites) probably pushing it for any Barovian PC. People who dungeon can still easily afford it. 78,000gp / year
5000/Week (Penthouse) Most players probably wouldn't want to pay this for very long, its really only a large convenience for those few people swimming in gold. Barovians, and most players wouldn't have this much 'extra' money. 260,000gp / year
Now the biggest issue that I can see that this sort of system adds, is that it doesn't promote a gold dump, but rather creates a need for expense, and people tend to try to earn more money when they have a new upkeep cost. This system really just makes people 'need' to dungeon more, which currently, probably happens a bit too much already.