Got here by an accident. I was absolutely astonished by your 100% native level English skills. I assumed you're a native until i heard single words in your Polish, showing clearly you're a Polish native. Never heard perfect English -> perfect Polish before in my 37 y.o life.
There is a shuttle bus running between Modlin Airport and Modlin station. Cost of ride is 6PLN. Also, there is a ticket valid for 75 minutes on route, Modlin Airport -> Modlin (bus) and Modlin -> any station in Warsaw which costs 15PLN. More on that here: www.mazowieckie.com.pl/pl/bilet-lotniskowy-oferta-specjalna (in polish, but Translate should do the trick) and here: en.modlinairport.pl/page/train . Hope you'll have a great trip!
Cost is mostly associated with distance travelled, so it depends on where your trip originates. I covered some websites in the video, this should be enough info to buy your ticket.