so this is where the opportunity can be exploited.
I don't think that you exploit the opportunity what you exploit is your roommates that you basically change extra and you live free on their expenses, to be honest, I wouldn't want to live like that, even if it's for free, plus there are of course risks, someone might leave and you might not find a replacement or because of the expenses they might all leave and then you're up paying $500 when all you need is a room, two months with the empty house in your example a year and you're basically losing money.
Then, there is the problem of finding such opportunities, if the owner of the house knows you can get a room rented for $125 that easily why would he offer you a discount knowing you're also a risky thing, so he would simply change $125 for each room, getting $625 instead of $500, and mitigating the risks of losing everyone at the same time.
And now think from the point of view of the other four guys, they could simply find themselves a 5 room house, rent with less money per person but they will have "your" room as a bonus so they could build their own men cave there, "
with blackjack and hookers".