I would also suggest checking both of your credit scores of you want to move out. Most apartments won't rent without a cosigner if your credit score isn't high enough. I moved out at 18 with my boyfriend(now husband)and had to have my parents cosign because I had hardly any credit! If your credit is low(due to not having any credit) I would suggest getting a credit card and just putting alittle on it each month that you know you can pay at the end of the month to boost your score. If you are already paying $650 a month in rent and her $150 in rent to your parents and the rents $550 you probably could afford it. I would discuss with your girlfriend her plans for the future(being 18 she may want to go to college eventually) so she may not be able to work as much later on or you may want to do some type of schooling too.Either way if you want to move out go for it. I did at 18 and though it was tough I always paid my bills on time and now at 28 I have a home that will be paid off in 10 years and have a paid off car and no debt with plenty of savings! So it can be done but like I said it can be very tough and you will have to scrimp a lot in the beginning!