Are you sure you aren't renting the "room" out at market value? Are you renting the room out to a friend or family sig cheaper than a typical roommate would pay.
Assuming you own the home: Just enter all numbers and expenses at 50%.
If you are renting the home from someone else, I would just record the same number of expenses and they will net out to Zero. I.e. If you are paying $900 a month rent and renting a room for $400, just record a rent expense of $400 per month. A roommate's rent is a "Return on Capital" provided rooms aren't being marked up to create a profit.
