Something is expensive if it costs a lot of money. Expensive things are usually very high quality, or have a brand name or are made by someone that people are willing to pay extra for. The opposite of expensive is cheap.
A plant or animal is extinct if there are no more of them alive anywhere in the world. You can only see extinct animals in books, or stuffed in a museum like the dodo in the picture.
Today, animals usually become extinct because people cut down all the trees and plants in the area where they live and they have nothing left to eat, or because people introduce a strange new animal that either eats them all so there are none left, or eats all their food and there is nothing left for them to eat.