What if I'm paying someone's mortgage and didn't know about it?

I've lived (rented) in a house with 3 other girls for a few months now for $600 (plus utilities) a month. But I'm starting to think I'm paying for more than just my stay. They told me rent for my stay was $400 and no more and that utilities should be no more than $80 for each of us. Obviously something has gone wrong here. I just don't think it's suppose to cost $120 more than they told me it would. And I don't think utilities would sky rocket this much, I mean one of our roommates stays with her boyfriend EVERY night. So is there anyway that I am paying for more than what they told me I would? Maybe their mortgage or giving them some profit that they didn't tell me about? And if they are making me pay them a profit or their mortgage, shouldn't they disclose that to me? Plus, my parents total for their home is $2200 a month including everything. And for us it's $1800 including everything and my parents house cost MUCH more then the one I live in now?

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