The two professional pet sitters I used both charged $15-$20 per visit, 30 minutes a visit.
Though if you have a dog, you have to pay that twice a day.
They would also pick up your mail, water your plants, alter light settings, etc.
If these friends/relatives are in the immediate neighborhood, and don't have to drive more then 2 miles, I would say $10 a day PLUS a gift is perfectly nice for a teenager. I would pay an adult more, and if they had to drive further, I would definitely pay them more. My good friend is coming to watch our 2 cats while we're gone. Since we have a free-feed bowl and water fountain, she only has to come 3 times, but has to drive 15 miles to get here. I plan on paying her $50, and I told her that if she wants to stay here (she's living with her parents while she's looking for a job), she can.