Where is the cheapest place to fly into this summer in Europe?
Impossible to answer. Right now London probably is the highest especially around the Olympics. After that it is a dice roll. Certain times, certain locations, depending on promotions, one place might be a little cheaper for one week and more expensive the next. Just have to constantly check.
And don't forget to add the cost of getting from whereever you fly inot to where you actually want to be.
And what price are you looking for? Bases on what I am reading, cheap this summer is higher then last summer