There are no such thing as 'Polish' rates. There are different market rates based on perceived quality of service (both in Poland and internationally). Yes, approx double 20zl is where my general price is, and the clients I have are happy to pay that and are happy with the level of service.
I'm not going around saying I'm the best/others are worse, the market is open. People are free to choose. I'm confident in what I offer and at the price I'm offering. Saying that, I have done lots and lots of pro bono work and work at significantly discounted rates, so I'm not driven by money either.
How do you work without the original text? Easily :)
My role is not to translate but proofread and edit. In that, there is communication between me and the the client. It's in this communication that we find out if the translation is good/bad and if the client's actual intended message is being communicated. You find quite often, the intended message is lost in the translation process or that careless translation rewords or rephrases originally intended messages. This is where communication comes in, and almost a decade's worth of experience.
Re: source language, I know Polish well enough thank you, but you're missing the point. The end result is communication in English, not necessarily English which was originally written in Polish. There's a subtle difference between the two, but a big enough difference that you can easily identify an English text which was originally Polish.