They can differ 1-2%, of course there are places where they can "rob" you even more.
Just check the National Bank of Poland (NBP - Narodowy Bank Polski) site at:
http://www.nbp.pl/Kursy/KursyC.html - for the most popular currencies before buying/selling.
today's average rates:
currency buys at sells at
1 USD 2,7057 2,7603
1 EUR 3,7356 3,8110
and today's average prices form 817 excange offices all over Poland
currency buys at sells at
1 USD 2.7332 2.8100
1 EUR 3.7290 3.8081
for countries like Serbia it's different
http://www.nbp.pl/Kursy/KursyB.htmlSerbia dinar 1 RSD 0,0470 - there you can find only average rate, so no idea at what prices they buy, probably not buying at all.