Are you kidding me? Why classic nintendo games are so expensive?
You gave us the answer: THEY ARE CLASSIC DUH!
Things which are classic worth more than "no classic" ones because they are not anymore under production.
It's pretty obvious, answer me Would you pay a high price for something that you can buy in another store?
No, because you know you can find it in another place; besides if you want to buy something that is hard to find you will pay more money, because there are only a limited quantity of those things and the company will not make more.
Basic knowledge of Economy ;)