I'd say a *minimum* of $200/week, you may want it to be higher, with the amount of work you're going to be doing. I've priced it anywhere from $215/week to $375/week. Usually infants cost more than toddlers, but that is more strictly applied in the larger places where they are concerned with the staff/child ratio. Not an issue for you since you'll be watching only the one child.
Not to be a stick in the mud, but you since you're going to be doing this full-time, you should consider (or at least be educated on) the licensing requirements here in Fairfax County, since you're in Reston.
My in-home daycare provider is licensed and while it is a bit of a pain to start it up, she said it is well worth it - besides the tax breaks, there is a built in support network of other providers and the staff in that agency. They have group meetings, etc. Also, consider your liability risk if you are not licensed, since the licensing requirement includes a safety inspection by the fire department, etc. Once the baby is on solid food, there is a USDA Child Care Food Program that is a subsidy for the provider as long as the child's meals meet the nutritional criteria. (My provider does this as well.)
Besides mixing the whole friends/business thing, you should also consider some very important points:
-have you defined how you're going to handle sick time - your's, your baby's and her child's? Notification, etc. I'm assuming that when/if your baby has something contagious that you won't take hers into the house, and of course you won't want to take her baby if it is sick, or if you are ill yourself.
-what about if you (or they) are going to take any vacation time? Most daycares charge a set amount weekly, regardless of whether the child is there or not, unless there is some kind of agreement of 1/2 the pay for when the child is on vacation for a straight week. Consider the difference between single days off in a week, versus the entire week (or more).
-when are you expecting to be paid? every Monday, the Friday before, once a month, etc.
-written contact information for them, information about allergies for the baby (even that there are none), and a medical treatment form authorizing you to have the baby treated in an emergency. There are generic ones that don't give excessive permissions, but every daycare I've been to has some form of this.
-lots of other points as the baby gets older (are you in this for the long haul, 2-3 years?, or longer?) addressing discipline, food/meals, etc.
My daycare provider has a whole 10+ page contract that spells out both our and her responsibilities and rights. It's a form one she got through her contacts, and very, very thorough. It protects both her and us and is very reasonable.
Like I said, not trying to put a damper on this, and I do have a friend who has done daycare for other friends and the friendship survived, but make sure you are educated on all of the factors going into this. It is not a small thing, you aren't talking about occasional babysitting here.