The best groceries stores are marked for you on the map (which you will received upon arrival). Every store has similar prices, for local and tourists. eggs 25 cents each, bread 25 cents each, marmalade 3.50 dollars, 350 grs processed cheese like 8 USD, ground coffee 8 USD, 250grs, chicken 2,50 USD each pound, fish 3 USD each pound, octopus 7.5 dollars one pound. Milk 2.75 dollars litter. Vegetables price may vary depending on the date you will stay in Isabela island. Most of these products come from the main land. Price depends on availability.
Food is pricier in the Island however it is cheaper to cook at home.