if I have a heron at the battlefield and I've already played a spell as my first card, then for my 2nd card I play bellows' breath repeated, how much do I pay if I play it repeated?

You pay 0 total.

Bellows Breath's base cost (1 Energy + 1 Power) plus its Repeat cost (1 Energy + 1 Calm Power) combine into a total of 2 Energy and 2 Power. Astral Heron's pending discount of [2][A][A] applies to the combined total — including the Repeat additional cost — reducing everything to 0 (Rules 356.4.f, 820). Since you declared you're paying the Repeat cost, it counts as "paid" even at 0, so the spell still executes twice. The Heron's discount is used up on this second card, and your third card is back to full price.