I based my charge on what would have had to be paid in a Boarding Kennel. And going by what both parties were happy with. I'd have been happy with that sum for only a long weekend!! Especially with no travel involved. In fact as the people we sat for, did the same for us on occasion, I only charged for the 3 weeks they took off overseas. I'm no business-woman and charged £300 the first time for the 3 weeks, house-sitting, grass cutting, plant watering, mail taken in..... plus 3 dogs and their 3 cats. Over the years since the first time, she didn't offer to increase that charge - and I didn't ask!!
Warning - it might seem like easy money, but it is a huge responsibility. If you have your own dogs, it may not be a good idea to bother about getting them together. Make sure you have a contact number for them, and the name/address of the vet they use (a note giving their vet permission to treat their dog if bought in by you, might be relevant too).