How much to charge a friend to sit her boxer for at least a month?

A friend of mine wants me to watch her dog for at least a month until she’s able to move into a new place. She offered to pay me $100 + food and treats. This means she’d be paying me $25/wk to watch her extremely hyperactive 60–70 lb. boxer. I made sure to ask if she would be able to watch the dog (at my house) if i needed to go out of town or do something. She agreed but is already making plans to go out of town the 2nd week of the month to a concert that i had planned on going to (i don’t think she knew I’d planned on going but that’s the exact reason I asked her that question, for these types if instances). She told me she wanted to come by most everyday and offer help and responsibility. I’m a college student graduating in May and there will be many days when i don’t need/want anyone at my house because I’ll be busy. Even when she is here spending the night or if she watches the dog here I’ll be the one paying the water and electric bill for her share. I would watch the dog even if she couldn’t pay me (and I told her this) but this is for a month and while I’m trying to help out a friend, I’m also trying to think of my own considerations and my time without being taken advantage of (I don’t think she’s trying to take advantage of me, I think she just doesn’t understand that this is a huge responsibility and is time consuming and pretty much puts me out of commission for a month). So, what is a reasonable price to ask for watching the dog for a month (minus food costs and treats), keeping in mind that I’m trying to help a friend but at the same time not putting myself out.