Right off the bat I am very skeptical about this article.
The facts in the article may be correct regarding Yahoo's prices, but it implies that Yahoo will turn over any email contents for a price. I doubt that is the case.
Unless Yahoo's lawyers are stupid, they would not allow any email disclosure to law enforcement unless Yahoo was served with a valid court order, subpoena or similar document.
The price list is just so Yahoo can recover its costs of complying with the court orders.