I've been on both ends of this. That's the problem when there is no writing.
My buddy got a divorce and needed someone to rent the extra room. Initially I had agreed to pay $400 + half of all the utilities/cable/etc. On the day I was set to move in we verbally agreed that I would just pay him a flat amount of $650 a month including everything. I considered it a little high for a small room... he probably considered it a little low considering what he was paying for the Condo. But regardless, that arrangement moved along smoothly for about a year.
Then his brother got out of the army and needed a place to stay. He started crashing on our couch. That same month his friend got back from a Eurotrip and also needed a place to crash... he started sleeping on the other couch. My friend started charging them both $150 each to rent the couches. Unfortunately my rent stayed the same... $650.
I wasn't very happy with this.
My perspective: I went from paying $650 to share a 2 bedroom condo with 1 person to paying $650 to share a 2 bedroom condo with 3 people + his girlfriend that was constantly there.
Their perspective: I was renting the room for $650 so the rest of the living situation was irrelevant.
I suddenly had to share a bathroom with two other guys, the condo was constantly a mess... they would both stay up drinking all night playing WoW in the livingroom and watching the big screen TV. Then they would sleep all day so by the time I got back from work at 5:00pm they were still sleeping and would get angry at me if I wanted to turn on the TV and watch the Celtics. "You have a TV in your room! Watch it in there!"... This lasted for a couple months until the electricity bill came...
Turns out, when you have two guys sitting on two desktop computers playing WoW and watching a 65 inch TV all night, plus doubling the dishes, laundry, etc... the electricity bill has a tendency to spike. The bill was for $500... my roommate (the original guy) demanded that we all chip in. I basically told him, "no way. You're crazy. I agreed to pay you $650 flat per month... I'm not going to get punished for you renting out the couches".
He thought he was right... I thought I was right. He basically said, "pay up or leave" and eventually I got so angry with the situation that I decided to respond by refusing to pay him any rent at all. He threatened to call the police and have me forcefully removed. I responded by calling the police and verifying that he would actually have to give me a written notice and at least a month to leave. To sum up, I ended up staying there for an extra month for free... and basically took $650 from him. I rationalized it as "back pay" for the 3 months I had to live with those slobs. Bottom line is that if nothing is in writing, you usually can't do anything about it...
We're back to being good friends again. As mentioned before... friends and business (or friends living together) is typically a really bad idea.