I am from a fairly low cost of living part of the country. We bought a small 3 bedroom 2 bath 'starter home' across town from the school. My roommate's "rent" was about $400, which paid 2/3 of the mortgage I think. My parents picked up the other 1/3 as my 'part'. I basically left it up to my roommate if she wanted a third roommate or not. I told her I preferred to only have one roommate, but she'd be the one sharing a bathroom so it affected her more. The 'going rate' on rentals in that area at the time was about $300/bedroom/month (meaning you could rent a 2 bedroom house for $600, a 3 bedroom was 900ish, etc). We said we were planning on charging $300 per bedroom. But if she wanted to be the only roommate, we'd only charge her $400. It was worth $100/month to her not to have to share a bathroom or deal with a third roommate. It was worth it to me and my parents to not have to deal with a third roommate, rent, extra wear and tear on the house, etc as well. If we'd taken two roommates at $300/month, the $600 mortgage would have been covered.
I also disagree with DVMD, though I do understand her point of view. A landlord is going to charge 2-3 students $900 for that same home where his mortgage is $600 and pocket $300 (after all, being a landlord is a job where your goal is to make money), so how is this any different? I guarantee that if the 3 bedroom home we bought was a rental with a regular landlord, the rent would have been at least $900-1,000. To live there with only two people, you'd each be paying $450-500 each to a landlord. If three people, $300 each. So my one roommate who paid $400 to split the house with just me was actually coming out ahead over living in a regular rental of similar size, and I was also able to pay significantly less. The math may work differently in another state. Other people might not view it as 'worth it' to live with the homeowner. But others are. We also didn't charge her any pet deposit or pet rent, there was no security deposit, etc since I knew her. We tried to make it beneficial for us but not a rip off or bad situation for her. Also, my roommate's parents were paying her living expenses, so it worked well. My parents sent hers a copy of all our bills (electric, gas, internet, tv; they paid half) and a statement for rent every month and her parents paid my parents. My roommate and I were not involved with it at all. I understand that we were lucky to be in such a situation. My roommate's parents actually bought the house from us when I graduated because she still had two years left. They did a similar arrangement with the new roommate.