Indeed Mykonos and Santorini from mid July to end of August are crazy expensive and there are no Greeks on the islands, almost everyone is international tourists, lol...But outside these days there are great deals to be had, especially if you book in advance. But even in peak July/August season you can occasionally find some great deals there....

In Paros, I suggest you stay in Parikia (capital city/port) instead, it is beautiful, cheaper, on the water/on the beach, and next to the bus system, so you don't even need a car.