If you look through f.e. justjoin.it, most Senior offers are between 20-30k pln(4,2-6,3k eur), most western companies will offer you 40 eur/h from my recent experience which would amount to 6,4k eur/mo. I'm personally looking for something between 45-50 eur/h and I'm not having much luck, but I already had few 40 eur/h offers within last few days(for .NET). Also Dropbox opened in Poland and offers something around 30-35k pln/mo, at least that's what a friend got there.