B.net Balance has never been used for subscriptions, only blocks of time. They haven’t allowed 30 day blocks for some time now - around two years ago this change was implemented. If you wanted 30 days purchasable with gold, you would have just had to buy the token from the AH and when it popped up asking if you wanted to apply it to your balance or as game time, you should have chosen the 30 days game time.
Just for future reference, Mirasol had done up a very handy dandy chart to explain what you can purchase for the different payment options.
