I watched my cousin's cat (at their appartment, so it involved hanging out, feeding, cleaning litter) for an entire semester in college for $75. My uncle was there on weekends, but still, it was 15 weeks, 5 days a week. Maybe I was cheap? lol
ETA: Oh wait, I missed that there is a dog involved. Make it $100.
If my sister does it I pay her about $20/ day to stay at my house. It we have a friend do it we pay $30/day but we have 3 dogs that are kinda of high maintenance.
Is she staying at your house for a week? When I was in college that would have been payment enough anything on top of it would just be a bonus. Maybe $100 and a case of beer in the fridge?