Only you can decided what private space is worth. It is like asking what a home is worth... I don't know, I can just tell you as a profession what people in the market are willing to pay for a home. If having your own room is worth the 6.2% extra then it is worth it. If not, load up the apartment with 6 people and have everyone share a room!! :)
In my opinion, nothing has been personal up to this point, I would pay an additional 6.2% to have my own room in a place.
On a side note, you want to make sure whatever you figure out, you put in writing between the roommates and have everyone sign a copy of it. Things that should be in there are stuff like:
1) What happens if the GF and BF split up?! If one moves out does the other have to cover both parts of the master bedroom rent or do all the roommates have to carry the slack?
2) What happens if one roommate is late on the rent? Are their penalties assessed by the roommates?
3) Does each roommate have to pay a deposit to the other roommates to make sure that there is enough cash to pay the rent if it is late. (Each roommate would pay a deposit equal to the amount of one month rent and held in an account to be used if the rent is late).
4) What about guests?! If you find a GF/BF and s/he starts staying over, at what point do you have to pay rent for them or are they allowed to stay as long as they want?!
Things like this should go into writing from the get-go.
