Suppose you decided to share an apartment with a girlfriend. She's going to count on you to pay half the bills. Telling her you only sleep and change your clothes there so you should only pay $50 and she should pay the other $700 isn't going to go over well. If you rented an apartment by yourself: telling the landlord that you'll hardly ever be there, so you only want to pay a fraction of what she's asking isn't going to work.
My parents didn't charge me, but I didn't have plans to stay. I was going to college and only home vacations and summer. If your mom wanted to rent out your room, she could probably get more than $150 a month. I rented out a room last summer and got $100 a week.
Your *stuff* is there, so Mom can't use that room for a hobby room or a guest room. She can't rent it out to make extra money. You have a place to go if you and bf have a fight, or when you break up. Boyfriend or his parents haven't offered to have you move in there. I'm guessing Mom does't charge you for laundry, hot showers or an internet connection, which would cost you money anywhere else.
Where I live, a 5' x 15' storage locker costs $100 a month and it isn't climate controlled.. No showers, internet, or sleeping facilities included.
I own a house. (No mortgage, it's paid for) I wish my expenses were only $150 a month. The property taxes come to more than that.