Beyond the free trial and the 30 days that comes with the purchase of the game, theres no way to continue playing without making some sort of payment to the company. Sorry.

However, there is an option for people who dont have credit cards and therefore cant set up a regular monthly payment. There are game time cards you can buy from various retailers(gamestop, amazon, etc) that work as a pseudo-subscription. You buy the card and add the code to your account, it adds 30 or 60 days to how long you can continue playing.

Hope that helps, best of luck.