Please be aware there is only one ATM in Isabela and there is no guaranty it will work during your stay.
In most places you can pay with cards. In some places there are high fees for transactions.
Transit control card
At the airport of Guayaquil o Quito you need to pay 20 USD for transit control card.
Galapagos entrance fee
Upon the arrival to Galapagos you will have to pass the immigration control and pay Galapagos entrance fee 200 USD per person.
ONLY CASH, there is no ATM at the arrival.
Isabela entrance fee
10 USD per person upon the arrival. ONLY CASH