The money (500-600 EUR per day) I get here locally is very much an outlier. I've heard only of a few other people who make that much, and they're all in my area of specialty (big data - meaning Spark, Hadoop etc. - on important projects for huge corps). So I wouldn't really count on someone just off the plane being able to land such a deal. Half of that is not too hard to get though for a techlead or a good senior in a hot field or perhaps even just in Java for a big (=difficult) backend project. Also, importantly, if you work as a contractor, the taxes are just 19%, which is amazing compared to most western countries.

Poland is by far the biggest market in the region (and Warsaw is by far biggest market in Poland, but Krakow, Wroclaw, Gdansk and Poznan are also viable) and I'd recommend moving here if you're eyeing EE. However, the really good deals here are all in contracting (you simultaneously get higher rates AND lower taxes) and I don't know how that works with visas and if it's even doable... Sorry, I just haven't met any non-EU foreigner here on a contract yet.