My thought is the Battle Packs have two "semi-randomized" decks, so the extra $2 your spending isn't really worth the extra content. The semi-random decks won't contain rares, so the extra $2 isn't going to get you extra value in the form of chances to pull money rares. It will just get you more cards. For most players, this isn't enough incentive. Spend another $2 beyond the Battle Pack, you'd get a third booster and another chance at a quality rare. However, in your situation where you're trying to teach your 12-year-old-son, this is a good way to see some color-specific mechanics and cheaply play around with color combinations.
Another thing you might consider is the Deck Builder Toolkit. Similar to the Battle Packs, for around $20 these contain 125 semi-random commons and uncommons, 4 packs from various sets, and 100 basic land (20 of each type). From a cost perspective, you'd be looking at $0.11 per card if you don't put cost on the basic land. But once again, the semi-randomized cards reduce overall value since there is no chance for money rares. The extra $4 gets you more cards, but not more value.