I am traveling to Greece in July for 20 days. How much spending money do i need?

I have already calculate my accommodation and ferry/bus cost and they come in at around 950$ CAD. We are spending time in Athens, Santorini, Mykonos, Naxos, and Corfu. Just wondering if anyone knows roughly how much spending money I will need for food, going out, and doing a bit of sightseeing? ..i am trying to spend the least amount possible!!