The cost per month for the Standard 180 day and the Entry Subscription are the same price. Although, with the 180 day, you are paying for 6 months up front.

From a business standpoint, I believe part of the reason they don't allow you to select more than 30 days using the Entry Level Sub, is because of the Veteran Rewards. By choosing to pay the lower price, you cannot obtain all Veteran Rewards all at once.