What would it cost to board the animals ? Figure at least that.
The animals dictate the price IMO. Boarding is expensive. House sitting is one thing. Animal sitting is another. My house doesn't need to be sitted so to speak. My belgain does. I prefer him not to be boarded, so I gladly pay a trusted person a majority of what I would have paid to board him to take care of him. $20 is way cheap. If he set the price so be it. I'd definitely tip if all is good on return.
... The last time he sat for us he wouldn't take any more money so I am thinking of maybe a gift card or something.