I petsit for several families. The one had 3 very low maintenance dogs (basicallly feed them twice a day, walk them several times a day), but it was WAY out of my way to work (about 20 minutes the opposite direction if I didn't hit traffic so it took me 45 minutes to an hour to get to work every morning and even longer to get home due to traffic). I stayed at their house and they paid me $30 a day. They asked me to do it a second time, after they lost the older dog so they were down to 2, and paid the same. This summer I will be house/dogsitting for them for about $50 a day.
This past weekend I dogsit for a family with 2 dogs and a cat (though I didn't have to do anything with the cat) for one night. Very easy dogs, fed them twice, opened the back door for them to go outside every once in a while. I was there for maybe 15 hours and they paid me $50.
I always leave it up to them. Both families allow me full access to their homes (any food, computer, phone, etc.) I think it depends on the value it is to you and how good she will be. If you know she will go above and beyond (rather than just feed, water, and let them inside/outside), then pay a little more. If she will just be doing the basics, then pay a little less.