Ok, I noticed that you get 75 crowns when you start the game. Even if you buy a monthly membership, you do not even get some crowns monthly to go with your membership; therefore, if I want to spend the crowns, what can I buy without purchasing any more?
Personally, if you are buying a membership, you should get some crowns with the purchase depending on the length of the membership bought. Therefore, if you have a monthly membership, you could save up crowns for a purchase in the future.
Granted, KI is in business to make money, but this would likely keep people playing longer if they do not want to buy the crowns separately.