Are you joking?! Anybody half decent in Warsaw is going to charge 150% to 200% of that price.
If he goes through an agency he'll certainly have to pay a lot more. There are, however, translators who charge the rate I quoted (which is what agencies pay translators). As for Warsaw, well it's not the centre of the universe, is it?
I charge per hour for proofreading. That way the better the text, the cheaper it is for the client. I avoid books, though.