Help!! Lots of roomies!!?

Ok so me and my husband have a 5 bedroom house. We don't have any kids so we decided to rent out our rooms to help us save some money. My husband has 3 friends that want to move in and my mom already lives with us. At the moment my mom pays 400 and we pay the rest and all utilities. The total house payment is 935. My husband told the guys 280 each. Obviously i want to lower my moms rent. Her room is bigger and downstairs where it stays cooler. She knows how much our house payment is. I think she should get the better deal because she's my mom and the guys are just my husbands co workers. Although the only reason we are moving ppl in is to help us save. Our approx total for rent and utilities without my mom would be 550. So, what is fair to charge my mom?