Oquijan's links above will take you to thorough discussions of this question. For those who don't know how to follow these links, highlight either one of them carefully, and then right click with your mouse.

If for some reason that does not work for you, the answer is that subscriptions are for a fixed period of time -- not a fixed number of cards. I highly recommend the 4-cards-per-day VGHD subscription for $34.95 which comes out to about 29 cents per card. The first time you take that deal, you get 6 free bonus credits. If you're close to getting all the VGHD cards, ideally, you should stay away from subscribing, booster packs, or buying for credits until there are at least 101 VGHD cards that you don't own. At that point, take the 4-card-per-day subscription for only one month, and be sure to cancel before it renews. By the time your month is up, there should be about 19 additional new cards released (not counting the one VIP card, and assuming the current rate of releasing cards continues), and you will have gotten all 120 in one month's time (121 counting your free VIP card). Congratulations, you are now "maxed out," and the hardest problem you will have to face is waiting until there are again enough cards you don't own so you can do it again for the lowest average price per card.

To apply a similar formula to a six-month subscription, you should wait until there are about 56 VGHD cards that you don't own. Then take the six-month one-card-per-day subscription. By the end of the six-month period, there will have been about 124 new cards released (not counting the six new VIP cards, again assuming they keep up the same release rate), and by the end of the six months you will have acquired all 180 cards (186 counting your 6 free VIP cards). Note that these numbers are estimates and are not guarantees. Cancel subscription, wait, repeat.

If you didn't play the numbers to your maximum advantage, you still got a heap of cards for a darn good average price. Enjoy.