taxis to and from the airport in Warsaw,
From the airport to the station 50-60zl, sometimes a little less. More at night or on Sunday/public holidays. If you have Uber, it's a little cheaper. Only use radio taxis (ones with the name of a company written on the roof sign). They all display the price in the window - in Warsaw it's usually between 1.80 and 3.20 zl per km, mostly 2.40.
If someone approaches you at the airport whispering "taxi", ignore them. Either get one at the taxi rank or order one from either of the firms who have stands inside.
In Mragowo, ask your hotel who they use and stick to them. Most are honest.