and for instance Krakow is more expensive than Berlin when salaries in Germany are at least twice as high as in Krakow
But bear in mind that unemployment in Berlin is really not doing well - the city is bankrupt, there's a lot of youth unemployment (one statistic I found suggests 1/3 young people are on welfare) and while it's very popular with tourists, it's got very little in the way of manufacturing/production/etc.
You can't compare the two - Krakow doesn't have the same problems that Berlin has. Young people in Poland often move to Krakow to work - with Berlin, they move away.
It's the same reason why property is more expensive on the Polish side of the Oder/Neisse - there's no shortage of work in Poland, but a dire lack of work in Germany.