How much to charge for rent for this girl? ?

Me and my bf have a condo and have an extra room that is fairly large for her. The problem is she's pregnant and I'm pregant too, and her babies father left her. I told her he doesn't have to pay but she insists.. Would having her pay 15% of electricity, eater and Internet/landliner be too much? And I mean them all together.. Or is it too little? She doesn't have a lot of money so I was thinking 70-200$ a month? What should I do!!!?