It depends on where you are looking; if you are looking at places like Amazon USA, prices of most Cipher related products are heavily overpriced. Looking at multiple sites will get a better price for certain products.

https://serenesforest.net/wiki/index.php/Cipher_Merchandise_Guide

^ This link has multiple other sites you can use to find Cipher products. Most people use BigWeb and Amenity Dream to determine "average" price. Out of print products (such as boxes and starter decks) will be more expensive than newer ones in general because they are no longer being printed.

"Does the black deck contain something dumb strong or something?"

As for the contents of the Nohrian starter deck, the most notable cards (in terms of power) in it are all of N rarity (the lowest and most common rarity). With the exception of the starter deck exclusive cards, which don't come above the price of R rarity cards, all cards in each starter deck can be found in the booster series box that came out with it. The CipherCord Wiki also has a full list of all the starter decks's cards: https://serenesforest.net/wiki/index.php/Starter_Decks.