Is Astral Heron for the first or second card played? The discount

The second card. Astral Heron counts as your first card played, and its trigger fires after it's finalized on the battlefield, so the discount applies to your next card — the second card you play that turn (FAQ #11863, #11610).

Note the timing: if you play another card before Astral Heron that turn, it would be the "first card," and the discount would then hit whatever you play after Heron resolves. It never discounts itself.